[FOLLOWUP][BUGFIX] Do not override hidden UC fields in user settings

Check if there is actually a user TSconfig available before using it.

Resolves: #77263
Releases: master, 7.6
Change-Id: Iafc58866325f000077f46f9e77fafb3c80069fe6
Reviewed-on: https://review.typo3.org/50100


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>