Passing Null Parameter from form to query

I have a form 'frmMultiSelectHistory' that has two unboound fields 'StartDate' and 'ExDate' that are used to select dates. There are times when dates have selected values they work like a charm. For example,

Between [forms]![frmMultiSelectHistory]![StartDate] And [forms]![frmMultiSelectHistory]![ExDate]

However, when the dates are NULL, I still need to be able to have the form's underlying query run. Is there a function that checks for NULL and if so, replaces it with a wild card '*'

I tried several things and haven't had much success.

Any thoughtsStart Free Trial