The docstring for svn_log_message_receiver_t says:
* If @a changed_paths is non-_at_c NULL, then it contains as keys every path
* committed in @a revision; the values are (@c svn_log_changed_path_t *)
* structures (see above).
This edge case requires special handling in the callback. It would be better to
pass an empty hash in this situation, so that the caller can just iterate over
the paths in the hash table without checking it for NULL.
See http://www.contactor.se/~dast/svn/archive-2004-07/0840.shtml for brief list
discussion of this issue.