[9.x] Fix factory breaking when trying to determine whether a relation is empty

[axlon]

This fixes a bug introduced in #45118. When a model's singular relation is loaded before the model is stored (e.g. in a afterMaking callback). The factory will try to call isEmpty() on it which will cause an error.

BadMethodCallException : Call to undefined method App\Models\OtherModel::isEmpty()

[WendellAdriel]

@axlon that's a nice catch, I didn't expect this since it's something that was not covered in the tests.
I'm not sure if that's mandatory but I would say that adding a test case covering this issue that you fixed would be a good idea.
Also, sorry for the bug introduced with my PR 😅