[BUGFIX] Exception in FileMetadataOverlayAspect

It is possible that PageRepository::getRecordOverlay() returns NULL which
throws an exception as ArrayObject::exchangeArray() either needs an
array or an (array-)object.

The fix adds a small check for the Result of the PageRepository function.

Change-Id: I248125154220e6250c4a2a6fdf29759211bf214b
Resolves: #54169
Releases: 6.2
Reviewed-on: https://review.typo3.org/25914
Reviewed-by: Steffen Ritter
Reviewed-by: Oliver Klee
Reviewed-by: Frans Saris
Tested-by: Frans Saris
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer