[BUGFIX] IdentityProperties were not set

After submitting an invalid form twice, extbase tries to
create the related submodel instead of edit.
This is because of the missing __identity part
for the related submodel

Resolves: #46185
Releases: 6.0, 6.1, 6.2
Change-Id: If3ec15b9eff0fc8d9a7dc682518cbfd72bb4665b
Reviewed-on: https://review.typo3.org/21101
Reviewed-by: Stefan Neufeind
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring