set_profile_flags(): allow named profiles without attachment

Named profiles can come without an attachment path specified, for example profile foo {...}

This patch fixes set_profile_flags() to actually allow that instead of erroring out.

References:

• https://bugzilla.opensuse.org/show_bug.cgi?id=1096269 comment 3 and 4
• https://bugs.launchpad.net/apparmor/+bug/1775591

This fixes a regression in 2.13, therefore I propose it for 2.13 and master.