ValueError: revision committer_date must be None if committer is None.

Sentry Issue: SWH-LOADER-BZR-6V

ValueError: revision committer_date must be None if committer is None.
(1 additional frame(s) were not displayed)
...
  File "swh/loader/bzr/loader.py", line 340, in store_data
    self.store_revision(self.repo.get_revision(rev))
  File "swh/loader/bzr/loader.py", line 427, in store_revision
    parents=self._get_revision_parents(bzr_rev),
  File "<attrs generated init swh.model.model.Revision>", line 19, in __init__
    import collections
  File "attr/_make.py", line 2975, in __call__
    v(inst, attr, value)
  File "swh/model/model.py", line 1125, in check_committer
    "revision committer_date must be None if committer is None."

Loading failure, updating to `failed` status

eg. happens while loading https://code.launchpad.net/~mikel-martin/opencart/mp