Do not set null fields

aleksis/apps/paweljong/schema/event_registration.py

@@ -107,7 +107,7 @@ class SendEventRegistrationMutation(graphene.Mutation):
 
         # Store contact information in database
         for field_name in ["date_of_birth", "sex", "address", "mobile_number"]:
-            if event_registration["person"] is not None and event_registration["person"][field_name] != "":
+            if event_registration["person"] is not None and event_registration["person"][field_name] is not None and event_registration["person"][field_name] != "":
                 if field_name == "address":
                     for addr_field in ["street", "housenumber", "postal_code", "place"]:
                         setattr(person, addr_field, event_registration["person"]["address"][addr_field])