Remove None reviewers from validation

This was caused by an issue where the "---" (None) reviewers were
causing the validation to fail, meaning that you had to fill out n-1
reviewers in the submission page in order for the save to go through.

The solution was to only check for duplicates with reviewers that were
selected.

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

hypha/apply/funds/forms.py

@@ -300,7 +300,7 @@ class UpdateReviewersForm(ApplicationSubmissionModelForm):
         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
         ]
 
         for field, role in self.role_fields.items():