[BUGFIX] Ensure copied records keep l10n_state behavior

This change adds a test for localized pages that have
a l10n_source "custom" value (e.g. "pages.author").

When copying the page and its translation,
the l10n_state field is then set to "source",
which is wrong.

copyRecord() sets l10n_source=0, however DataMapItem
expects "null". The patch makes DataMapItem understand
0 as value for l10n_source as well.

Resolves: #88974
Releases: main, 12.4
Change-Id: I326765d92ea52bb49d94bc4965b998b993b5e673
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80572


Reviewed-by: Stefan B�rk <stefan@buerk.tech>
Tested-by: Stefan B�rk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>