Don’t attempt to store domain as empty string

When there is a uniqueness constraint on a DB column you can still have
multiple null values.

You can’t have multiple empty string values.

We are trying to save the domain as empty string when creating or
updating a new branding. This means that it’s currently not possible to
create or update a branding with no domain, because the uniqueness
constraint is violated.

app/main/forms.py

@@ -728,6 +728,8 @@ class GovernmentDomainField(StringField):
     ]
 
     def post_validate(self, form, validation_stopped):
+        if self.data == '':
+            self.data = None
         if self.data and not self.errors:
             self.data = AgreementInfo(self.data).canonical_domain