fix: handle empty ManyToMany relationships in UIConfigItemSerializer

- Filter out None values from Tag.loc_msg relationships to prevent LocalizedString.DoesNotExist errors
- Fixes /api/2/projects/1/uiconfig/ endpoint when tags have no localized messages
- Resolves issue where empty ManyToMany queries return [None] instead of []

Fixes: LocalizedString matching query does not exist error in uiconfig endpoint

Author: hburgund

roundware/api2/serializers.py

@@ -552,6 +552,8 @@ def to_representation(self, obj):
 
         # display localized tag text in addition to tag_id
         tlm = Tag.objects.filter(pk=result['tag_id']).values_list('loc_msg__id', flat=True)
+        # Filter out None values from empty ManyToMany relationships
+        tlm = [id for id in tlm if id is not None]
         lm = _select_localized_string(tlm, session=session)
         result['tag_display_text'] = lm