[BUGFIX] No category mounts set means access to all

Feature ##59063 introduces a inconsistent change in behaviour.

When rendering the category tree the BeGroup category mounts are only
respected when at least 1 category is selected else you got access to
all. See DatabaseTreeDataProvider::isCategoryAllowed()

But the check/restriction on the TceForm sees no mount points set
as no permissions at all.

This patch changes the check on TceForm that if no mount points are set
there is no restriction like rendering the category tree.

Resolves: #61067

Change-Id: I7129edd49b96326fb557622cb928a36d5a403b61
Reviewed-on: http://review.typo3.org/32246
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>