GraphQL: Errors with nullish ManyToOne-Relation when using new ResolverFactory

[jotwea]

Q	A
Branch?	main
Tickets	Closes #6083
License	MIT
Doc PR	-

This PR introduces a new the test case especially for properties and Many-To-One-Relations being null. It fixes the issues relevant for 3.2 and additionally provides a fix for the nested collection feature which was recently introduced on the main branch. There is another PR https://github.com/api-platform/core/pull/6092 that contains only the fixes relevant for 3.2.

[soyuka]

you can target the lowest branch, 3.x are merged back

[soyuka]

thanks cherry-picked