Remove None reviewers from validation for batch change

This is the same change as ee286ac5, but for batch reviewer change.

Issue ots/clients/smex/hypha-tracker#2: Can't assign Staff
as reviewers of applications

hypha/apply/funds/forms.py

@@ -426,7 +426,7 @@ class BatchUpdateReviewersForm(forms.Form):
         role_reviewers = [
             user
             for field, user in self.cleaned_data.items()
-            if field in self.role_fields
+            if field in self.role_fields and user
         ]
 
         # If any of the users match and are set to multiple roles, throw an error