[BUGFIX] Properly check if language was not fetched from UC

The date-time-picker component now checks properly if the backend
user's language could not get fetched from its UC by checking loose for
a falsy value.

Resolves: #99067
Releases: main, 11.5
Change-Id: I3ba104afde2321480099e96beff6ab77ecb126a2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76568


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>