[BUGFIX] #5173 PHP crash in the BE module if no import path is set

Change-Id: I309ffe1ff3068f750a4168e6bc4c19480a95fd53
Reviewed-on: http://review.typo3.org/34704
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>