It's possible, but takes a bit of sneakiness to get it to work.
Define a calculation field such as this to use in place of the first value list field:
If ( IsEmpty ( FirstValueListField ) ; "**All**" ; FirstValueListField )
Clear the Do not evaluate if all referenced fields are empty.
Now define a new calculation in the table serving as the source of values for your conditional value list:
List ( "**All** ; MatchFieldInCurrentRelationship )
Now use these two fields in place of your current relationship and an empty field will match to all values in the value list.