[BUGFIX] Ignore userTSConfig if not set in getModTSconfig()

It can happen that settings that are not defined in
userTSConfig can overwrite pageTSConfig settings
with a null value because of the change in #51007

This case is now checked so that in this case
the original pageTSConfig value is preserved.

Resolves: #51871
Releases: 6.0, 6.1, 6.2
Change-Id: I3fa356a1e5aa20716a6bc83f7c76b8b755fc0bbf
Reviewed-on: https://review.typo3.org/23703
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters