Stored procedure with parameters and dates

I have a database that has a column for 'filedon' which is a datetime column (SQL 2008)

I want to do a SP that will query the table and bring the records that match 'filedon' BEWTEEN two dates. This is simple. Here is the issue:

My search page has a 'from' and a 'to' fields:

1. If user leaves those fields blank then ALL results should be returned.  In my ASP I need to enter default dates for the parameters so I enter '01/01/1900' and '01/01/2050'  if user leaves it blank and those are the dates then it should return ALL records.

If the user DOES enter a value in the search page then the SP should return ONLY records that match that criteria (No nulls).

Here is a test version of the SP to try out:

ALTER PROCEDURE [dbo].[delete_report_byuser_01_results] 
( 
@Firmid INT,
@filed_start DATE, 
@filed_end DATE

)
AS
SELECT 
        a.Id ,
        a.CaseId ,
        z.RcptNo ,
        z.Filedon ,
        z.Approvedon ,
        z.Validtodate,
        s.description AS casestatus
FROM    Cases a
        LEFT JOIN Users AS f ON a.EmpId = f.UserId
        INNER JOIN dbo.admin_casestatus AS s ON s.idstatus = a.Archived
        LEFT JOIN Users AS d ON a.AlienId = d.UserId
        INNER JOIN FilingType AS k ON a.Processtype = k.FilingTypeId
        LEFT JOIN Employment AS L ON d.UserId = L.UserId
                                     AND L.CurEmp = 1
        LEFT JOIN FirmAddresses AS m ON a.FirmAddressId = m.Id
        INNER JOIN Processcatalog AS e ON e.ProcesscatalogID = a.Process
        INNER JOIN Atts AS J ON a.Id = J.CaseId
        INNER JOIN Users AS I ON J.UserId = I.UserId
        LEFT JOIN PreferenceType AS o ON o.PrefTypeid = a.PrefType
        LEFT JOIN Partiescont AS v ON d.UserId = v.PartcontId
                                      AND Relation = 'spouse'
        LEFT JOIN Users AS h ON v.UserId = h.UserId
        LEFT JOIN Activities_Misc AS z ON z.act_misc_id = a.Defaultrecpt
              
        
WHERE   a.FirmId = @Firmid
 AND ( z.Filedon BETWEEN @filed_start AND @filed_end ) 

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I run it with the following paramters to simulate user entry:

EXECUTE [delete_report_byuser_01_results] 
	@Firmid = 2, 
    @filed_start = '01/01/2015',
    @filed_end = '01/01/2050'

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It returns correct data. The problem is when user leaves it blank. Then it uses the default dates and takes out all the 'null' values.
The problem if I add 'null' to the WHERE clause like:

WHERE   a.FirmId = @Firmid
 AND ( z.Filedon BETWEEN @filed_start AND @filed_end ) OR z.Filedon IS NULL

Then it will return nulls also when I enter other dates. that are not the default dates entered by the ASP.

How can I modify the SP so that if the dates are different from '01/01/1900' AND /01/01/2050' then it should filter the results, BUT if those are the dates (which means user left them empty) then it should return ALL records.