perm check: forbid undefined/empty ACL path

perm check: forbid undefined/empty ACL path

to detect similar issues to that fixed in the previous commit early on
and without the potential for dangerous side-effects.

root@pam is intentionally still allowed before the check in case such
situations can be triggered by misconfiguration - root@pam can then
still clean up the affected configs via the GUI/API, and not just via
manual editing.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>