Add return type explicitly

[franmomu]

To avoid deprecation warnings like: https://github.com/sonata-project/SonataAdminBundle/runs/6369999365?check_suite_focus=true#step:8:57

[derrabus]

Thank you @franmomu.

[acrobat]

@derrabus would it be possible to release this fix? 🙏

Edit: I've created a PR to fix another return type warning/deprecation #108

[derrabus]

Done. ✅