Fix a bug in the authz parser where using a group with no members in an access entry was treated as an error instead of being ignored.
* subversion/libsvn_repos/authz_parse.c (add_to_group): Allow NULL user, to create empty groups. Update docstring. (expand_group_callback): Handle the case where a group has no users. (array_insert_ace): Ignore ACEs for empty groups.
* subversion/tests/cmdline/svnauthz_tests.py (svnauthz_empty_group_test): Remove XFail decorator. Extend the testcase to test recursive empty group expansion.