Fix bug #346431 by allowing log._filter_revisions_touching_file_id to be called on an empty branch.
* bzrlib/tests/test_log.py: (TestGetRevisionsTouchingFileID.test_empty_branch_unknown_file_id): Reproduce bug #346431 by trying to log an unknown file in an empty branch.
* bzrlib/log.py: (_filter_revisions_touching_file_id): Initialize next_keys so we can 'del' it later.