[BUGFIX] User settings do not obey setup.override

Add a missing isset() check whether a setting has been overridden
by user TSconfig.

Resolves: #57690
Releases: 6.2, 6.1
Change-Id: Id1290b2af85061051ce0cedff4a0be96a91c4dce
Reviewed-on: https://review.typo3.org/29239
Reviewed-by: Renzo Bauen
Tested-by: Renzo Bauen
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind