[BUGFIX] Type checking at PageRepository::getFileReferences() call

Verification of parent element ($element) type before
PageRepository::getFileReferences() call
in FilesContentObject::handleFileReferences
to avoid PHP error in case of null value

Resolves: #64239
Resolves: #62616
Releases: master, 6.2
Change-Id: I8b89710af97eb37eee9518392e89ddeceb5b9f57
Reviewed-on: http://review.typo3.org/36480


Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>