How to handle null/nothing/blank values on MS Access 2010 form controls within VBA?

I have a need for a "track-changes" capability in Access 2010. I ran across this script which should do the trick with a little work.

It's clear and understandable but it doesn't quite yet handle all of the cases I need for textboxes. (I'll implement other controls later.) This is the pertinent structure as written. It's called as an event procedure for the Before Update event on a form:

Const cDQ As String = """"

Sub AuditTrail(frm As Form, recordid As Control)

    'Track changes to data.
    'recordid identifies the pk field's corresponding
    'control in frm, in order to id record.

    Dim ctl As Control
    Dim varBefore As Variant
    Dim varAfter As Variant
    Dim strControlName As String
    Dim strSQL As String

    On Error GoTo ErrHandler

    'Get changed values.

    For Each ctl In frm.Controls    
        With ctl
            'Avoid labels and other controls with Value property.
            If .ControlType = acTextBox Then
                If .Value <> .OldValue Then
                    'Add record of change to logging table   
                End If     
            End If     
        End With   
    Next   

    Set ctl = Nothing   
    Exit Sub  

    ErrHandler:
        'Handle error
End Sub   

This works as long as the values within the text box are not blank. If one of .Value or .OldValue is null (or nothing -- I'm confused on this point) then the inequality fails.

I tried updating the part within With ctl to the following. This allows me to log changes for which one of the values (before or after) was null. The rest of the commented-out stuff is my attempt to record all of the cases where the value is null, but doesn't change. In other words, the IsNull(ctl) lets everything through, but I couldn't figure out how to filter the cases of equality back out again.

If .ControlType = acTextBox Then
    If IsNull(ctl) Or .Value <> .OldValue Then
        'If Not (Len(.Value & vbNullString) = 0 And Len(.OldValue & vbNullString) = 0) Then                 

            'If oldValueIsNull Then
            '    varBefore = "--NULL--"
            'Else
            '    varBefore = .OldValue
            'End If

            'If newValueIsNull Then
            '    varAfter = "--NULL--"
            'Else
            '    varAfter = .Value
            'End If

            'Add record of change to logging table

       'End If
    End If
End If

Any help is appreciated.

If you are happy to just avoid the whole null issue, which may not suit everyone, you can say

If .Value & "" <> .OldValue & "" Then 

Which will compare strings.