[BUGFIX] Use less strict check for empty translated values

When translating values in EXT:form, we must ensure to
handle translated values correctly. Prior to this change,
the native PHP function "empty()" was used to determine
if a translated value is valid. This, however, generates
wrong translations for valid empty values, such as "0".
Therefore, the native function has been replaced by a
helper method in TranslationService.

This fix is a potential change of behavior. Translated
values that were previously interpreted as "empty" might
now be rendered as-is. This can possibly lead to a
different rendering result in Frontend forms. Therefore,
this change is not backported to previous versions.

Resolves: #91833
Releases: main
Change-Id: I373ca97f91b97495f605ad3344b355539966a371
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74992


Tested-by: core-ci <typo3@b13.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: waldhacker <hello@waldhacker.dev>