CVS loader is failing with empty modules

It exists CVS repositories with modules containing no changesets. This currently triggers the following error while an empty snapshot should be produced instead (as the git loader does).

Sentry Issue: SWH-LOADER-CVS-3M

StopIteration: 
  File "swh/loader/cvs/loader.py", line 616, in fetch_data
    data = next(self.swh_revision_gen)

AssertionError: 
  File "swh/loader/core/loader.py", line 417, in load
    more_data_to_fetch = self.fetch_data()
  File "swh/loader/cvs/loader.py", line 618, in fetch_data
    assert self._last_revision is not None