[BUGFIX] Prevent crash in DatabaseIntegrityCheck

If a relation field (select / group) does not exist in db for
whatever reason, System -> DB check -> Database relations
crashes.

Resolves: #86167
Releases: master, 8.7
Change-Id: Id6a502d56b985f6410f40a1f547fe7b923e9d982
Reviewed-on: https://review.typo3.org/58201


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Tested-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>