Fixes: 1) Fix `resolve_recently_viewed` to handle empty UUIDs and avoid breaking queries.
Extra: Refactor imports in `graphene/object_types.py` and `serializers.py` for better clarity; Adjust minor formatting in `TokenObtainSerializer`.
Fixes: 1) Correct resolve_recently_viewed logic to ensure default behavior;
Extra: 1) Minor cleanup in resolve_recently_viewed for simplified return logic;
Fixes: 1) Add missing import for `transaction` in `signals.py`; 2) Fix incorrect handling of `pre_save` skip logic for non-existent users; 3) Correct conditional logic for email change handling to avoid unnecessary activation resets.
Extra: 1) Simplify mutation attribute validation in `mutations.py` by excluding specific fields earlier in the loop; 2) Improve code readability with minor refactors in both `signals.py` and `mutations.py`.
