Problem with empty strings and null values

I am using CF8 and a MS Access database to do a routine query. On my form page I have a several form fields that allow the user to query the database based on certain parameters (last name, email, city, state, etc.).

After I do my query I need to put whatever search parameters are entered into the form in a session object for use later in the application. The problem that comes up is, if the user leaves fields blank, when I put them into a session object they are being seen as empty strings and I need them to be seen as null. Here is the relevant code on the results page:

<!--- Create session objects --->

                    <!--- Last Name Session Variable --->
                  <cfif isDefined("form.Last")>
                    <cfset session.Last="#form.Last#">
                  </cfif>
                <!--- Email Session Variable --->
                  <cfif isDefined("form.Email")>
                    <cfset session.Email="#form.Email#">
                  </cfif>
                  <!--- City Session Variable --->
                  <cfif isDefined("form.City")>
                    <cfset session.City="#form.City#">
                  </cfif>
                  <!--- State Session Variable --->
                  <cfif isDefined("form.State")>
                    <cfset session.State="#form.State#">
                  </cfif>
                  <!--- Zip Session Variable --->
                  <cfif isDefined("form.Zip")>
                    <cfset session.Zip="#form.Zip#">
                  </cfif>
                  <!--- Gender Session Variable --->
                  <cfif isDefined("form.Gender")>
                    <cfset session.Gender="#form.Gender#">
                  </cfif>
                  <!--- Year Session Variable --->
                  <cfif isDefined("form.YearGrad")>
                    <cfset session.YearGrad="#form.YearGrad#">
                  </cfif>
                  <!--- Program Session Variable --->
                  <cfif isDefined("form.Program")>
                    <cfset session.Program="#form.Program#">
                  </cfif>
                  <!--- Teacher Session Variable --->
                  <cfif isDefined("form.Teacher")>
                    <cfset session.Teacher="#form.Teacher#">
                  </cfif>
                  <!--- School Session Variable --->
                  <cfif isDefined("form.School")>
                    <cfset session.School="#form.School#">
                  </cfif>
                  <!--- SpecServ Session Variable --->
                  <cfif isDefined("form.Spec_Services")>
                    <cfset session.Spec_Services='#form.Spec_Services#'>
                  </cfif>
                  <!--- Survey1 Session Variable --->
                  <cfif isDefined("form.Survey1")>
                    <cfset session.Survey1='#form.Survey1#'>
                  </cfif>
                  <!--- Survey2 Session Variable --->
                  <cfif isDefined("form.Survey2")>
                    <cfset session.Survey2='#form.Survey2#'>
                  </cfif>
                  <!--- Survey3 Session Variable --->
                  <cfif isDefined("form.Survey3")>
                    <cfset session.Survey3='#form.Survey3#'>
                  </cfif>
                  <!--- Survey4 Session Variable --->
                  <cfif isDefined("form.Survey4")>
                    <cfset session.Survey4='#form.Survey4#'>
                  </cfif>

<!--- Run query --->
<cfquery name="survey" datasource="ctesurvey_dsn">
                  SELECT *
                  FROM data
                  WHERE
                        <cfif isDefined("session.Last")>
                          Last LIKE '#session.Last#'
                              AND
                        </cfif>
                        <cfif isDefined("session.Email")>
                          Email LIKE '#session.Email#'
                              AND
                        </cfif>
                        <cfif isDefined("session.City")>
                          City LIKE '#session.City#'
                              AND
                        </cfif>
                        <cfif isDefined("session.State")>
                          State LIKE '#session.State#'
                              AND
                        </cfif>
                        <cfif isDefined("session.Zip")>
                          Zip LIKE '#session.Zip#'
                              AND
                        </cfif>
                        <cfif isDefined("session.Gender")>
                          Gender LIKE '#session.Gender#'
                              AND
                        </cfif>
                        <cfif isDefined("session.YearGrad")>
                          YearGrad LIKE '#session.YearGrad#'
                              AND
                        </cfif>
                        <cfif isDefined("session.Program")>
                          Program LIKE '#session.Program#'
                              AND
                        </cfif>
                        <cfif isDefined("session.Teacher")>
                          Teacher LIKE '#session.Teacher#'
                              AND
                        </cfif>
                        <cfif isDefined("session.School")>
                          School LIKE '#session.School#'
                              AND
                        </cfif>
                        <cfif isDefined(session.Spec_Services)>
                          Spec_Services = #session.Spec_Services#
                              AND
                        </cfif>
                              <cfif isDefined(session.Survey1)>
                          Survey1 = #session.Survey1#
                              AND
                        </cfif>
                              <cfif isDefined(session.Survey2)>
                          Survey2 = #session.Survey2#
                              AND
                        </cfif>
                              <cfif isDefined(session.Survey3)>
                          Survey3 = #session.Survey3#
                              AND
                        </cfif>
                              <cfif isDefined(session.Survey4)>
                          Survey4 = #session.Survey4#
                              AND
                        </cfif>
                  Email IS NOT NULL
                  ORDER BY Last ASC      
                  </cfquery>
**********************************
When I do a dump of the session objects, the ones that the user left blank are displayed as empty strings.
This is causing my query to return 0 results. How can I get these session objects to be seen as null values?

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