[TASK] Use better implementation for array check

To suppress PHP warnings the input parameter must always be an array.
If there is nothing to merge, the array_merge_overrule function
must not be called at all.

Change-Id: Ice369449da22ff62c09efa919272195bd141e836
Resolves: #63827
Relates: #63701
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/35398
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>