[BUGFIX] Return correct source record for relations

This change addresses a flaw when resolving ancestor ids of 1:n
relationships - resolving `l10n_source` pointer always was `0`.

Besides that now only non-empty `l10n_parent` or `l10n_source` are
returned as integer. Otherwise `null` is returned and signals that
a corresponding ancestor could not be resolved at all.

Scenario `localizeParentContentChainLanguageSynchronizationSource` of
`DataHandling\IRRE\ForeignField\Modify\ActionTest` was given already,
however `l10n_source` fields were missing in their TCA.

Resolves: #86141
Releases: master, 10.4, 9.5
Change-Id: Ia0e3d9accd5c1998f4fd971861262c158cc8f838
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/58049


Tested-by: core-ci <typo3@b13.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>