[BUGFIX] array_merge_recursive_overrule: __UNSET for array values

Using __UNSET on keys that hold array values does not unset them.
However the documentation does not mention this restriction.
Therefore this is considered a bug and fixed by this patch.

Resolves: #43874
Releases: 6.1, 6.0, 4.7

Change-Id: Ie9f96c6f608da7e91fec2bc639dba9f3bcc8f426
Reviewed-on: https://review.typo3.org/17096
Reviewed-by: Markus Klein
Reviewed-by: Wouter Wolters
Reviewed-by: Tobias Liebig
Tested-by: Tobias Liebig