Resolves#960
The update_project endpoint now properly handles clearing nullable fields
(due_date_at, area_id, priority, image_url) when set to null or empty string.
Previously, sending an empty string would not clear the field. Now empty
strings are explicitly converted to null for database consistency.
Changes:
- Convert empty strings to null for nullable fields in project update
- Add tests for clearing due_date_at with null and empty string
- Apply same fix to area_id, priority, and image_url for consistency
