[TASK] Superfluous comparison in RelationHandler

Fix superfluous comparison against boolean in
RelationHandler::readForeignField

Change-Id: I77f17dee6a14da7779dfe8e37bc73f33a3d02cb5
Resolves: #54048
Releases: 6.2
Reviewed-on: https://review.typo3.org/25735
Reviewed-by: Jo Hasenau
Reviewed-by: Michiel Roos
Tested-by: Michiel Roos
Reviewed-by: Markus Klein
Tested-by: Markus Klein