[common][serializers] Fix object existence check

The check worked with SQLite but not with PostgreSQL.
In any case, since we have a user object here, there's no
reason to not use it directly.
1 job for fix_common_serialization in 14 minutes and 44 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Build
passed #195358
docker-build
build_linux_36

00:14:44

64.0%