Refs #28147 -- Fixed loss of assigned parent when saving GenericForeignKey after parent.

[sarahboyce]

Follows on from "Fixed #33004 -- Made saving objects with unsaved GenericForeignKey raise ValueError."

Aims to allow saving a GenericForiegnKey after the parent (see test_save_generic_foreign_key_after_parent), without removing the ability to save the fk_field of the GenericForeignKey as None and that nulling out the GenericForiegnKey value (see test_assign_gfk_id_none).

We should also check if this doesn't introduce a regression as https://github.com/django/django/commit/519016e5f25d7c0a040015724f9920581551cab0 (fixed in https://github.com/django/django/commit/4122d9d3f1983eea612f236e941d937bd8589a0d).

The only solution I have found so far to prevent the regression is probably quite hacky/flakey. Look forward to the suggestions people have as to how it can be improved :+1: