[BUGFIX] Prevent exception in RTE filelinks upgrade wizard

When using the upgrade wizard for RTE filelinks and the record is
not an array it throws an exception.

To give the end user a better experience an additional condition
for checking that the variable is an array is added.

Resolves: #58898
Releases: 6.3, 6.2
Change-Id: I94ae6cc7272a63fd208267bf375b629cb5a4b2cc
Reviewed-on: http://review.typo3.org/31933
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>