[BUGFIX] Fix PHP warning in shouldFieldBeOverlaid()

PageRepository::shouldFieldBeOverlaid now properly checks for array-typed
values when mergeIfNotBlank behaviour is requested.

Resolves: #60998
Releases: master, 6.2
Change-Id: I9b536d2261eea6325e0f74365a3a4c85e37b1241
Reviewed-on: http://review.typo3.org/34523
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>