[BUGFIX] Exception if thumbnail does not exist

BackendUtility::thumbCode() tries to retrieve the extension
of a file object that might be NULL.

Add a check for having a valid file object.

Resolves: #54394
Releases: 6.1
Change-Id: Ia97fbb5fd3d9bd53c0d776a93969f9d059eebc4a
Reviewed-on: https://review.typo3.org/26446
Reviewed-by: Oliver Klee
Reviewed-by: Stefan Neufeind
Reviewed-by: Markus Klein
Tested-by: Markus Klein