[BUGFIX] Let FrontendGroupRestriction handle missing TSFE gr_list property

Check for an uninitialized frontend group list property in the
FrontendGroupRestriction and provide proper input to explode();

Change-Id: I807c0e1dc748ed1547a7dd937f0da69e69bb6e4c
Resolves: #77712
Releases: master
Reviewed-on: https://review.typo3.org/49704


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Julian Hofmann <info@julian-hofmann.net>
Tested-by: Julian Hofmann <info@julian-hofmann.net>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>