message: make sure there is a val to be set for existing filters
diff: @@ -737,7 +737,7 @@ def merge_extra_filters(form_data):
  return f['col'] + '__' + f['op']
  existing_filters = {}
  for existing in form_data['filters']:
  - if existing['col'] is not None:
  + if existing['col'] is not None and existing['val'] is not None:
  existing_filters[get_filter_key(existing)] = existing['val']
  for filtr in form_data['extra_filters']:
  # Pull out time filters/options and merge into form data
  