[BUGFIX] Admin panel extGetFeAdminValue() overrides are wrong

The current check does not properly consider the override values.
If a override is set, then its value has to be taken.
The current check only considered overrides which had
boolean value TRUE.

Resolves: #58339
Releases: 6.2, 6.1
Change-Id: Ib714432044c6a88fc92dcb8f36a4e37eb3412378
Reviewed-on: https://review.typo3.org/29730
Reviewed-by: Gerrit Mohrmann
Tested-by: Gerrit Mohrmann
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters