[BUGFIX] PHP warning in ext:documentation

PHP warning will be thrown if typo3conf/Documentation
does not exists. Add a check to prevent the warning.

Change-Id: Ia9def8bb5b36da888eba3817ded9b348f48c37a3
Resolves: #50841
Releases: 6.2
Reviewed-on: https://review.typo3.org/22893
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind