hit the is_valid method before stripping the special fields in credential view

awx/api/views.py

@@ -1231,6 +1231,11 @@ class CredentialList(ListCreateAPIView):
     serializer_class = CredentialSerializerCreate
 
     def post(self, request, *args, **kwargs):
+
+        # Check the validity of POST data, including special fields
+        serializer = self.get_serializer(data=request.data)
+        serializer.is_valid(raise_exception=True)
+
         for field in [x for x in ['user', 'team', 'organization'] if x in request.data and request.data[x] in ('', None)]:
             request.data.pop(field)
             kwargs.pop(field, None)