[BUGFIX] Apply empty values in language-overlay

Some default fields (e.g. tt_content.bodytext) can contain null values.
TYPO3 first fetches the data in the default language and then overlays
the rows data with the translation values. The overlay method inspects
each array item with the php isset() function. This validates not only
the existence of the array key, but also the values. null and
false values evaluated as false. Empty strings evaluate as true.
This leads to inconsistent output in the frontend.

The overlay now valides only the array key existence and applies also
empty values.

Resolves: #97616
Releases: main, 11.5, 10.4
Change-Id: I4b01c52e9ac7adde786b3395bce870bc0a354b58
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74837


Tested-by: André Buchmann <andy.schliesser@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: André Buchmann <andy.schliesser@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni...