MEDIUM: acl: remove the ACL_TEST_F_NULL_MATCH flag

MEDIUM: acl: remove the ACL_TEST_F_NULL_MATCH flag

This flag was used to force a boolean match even if there was no pattern
to match. It was used only by http_auth() and designed only for this one.
It's easier and cleaner to make the fetch function perform the test and
report the boolean result as a few other functions already do. It simplifies
the acl_exec_cond() logic and will help merging ACLs and patterns.