[BUGFIX] Exception on thumbnail generation in TCA

In order to test this you are needing a non-FAL
TCA file reference field. Just reference something
here, remove the reference in the filesystem again
and reload the TCA of the record. An exception will
be thrown, because the file isn't available anymore.

Fixes: #49009
Releases: 6.2, 6.1, 6.0
Change-Id: I6eccb3ef77de703551a64f52ab3904cead517c8a
Reviewed-on: https://review.typo3.org/21328
Reviewed-by: Markus Klein
Reviewed-by: Alexander Opitz
Reviewed-by: Ingo Pfennigstorf
Tested-by: Ingo Pfennigstorf
Tested-by: Markus Klein