[CLEANUP] File object

In the TYPO3\CMS\Core\Resource\File object the logic to load
metaDataProperties was implemented at multiple places but there
is also a method _getMetaData() that handles this logic.
The method _getMetaData() should be used instead of duplicating
the logic at multiple places.

Additionally some code is removed, which checks the
metaDataProperties member for NULL values, which can never
be the case.

Resolves: #66878
Releases: master
Change-Id: I01939a66aa58629f2c6dc08656647375460cc4e9
Reviewed-on: http://review.typo3.org/39429


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>