message: Fix return type for ledger `get_public_domain`
diff: @@ -477,15 +477,15 @@ class Entry:
  buffer = _byte_read_safe(self._file, LEDGER_DOMAIN_SIZE)
  self._public_domain_size = to_uint_64(buffer)
  - def get_public_domain(self) -> Optional[PublicDomain]:
  + def get_public_domain(self) -> PublicDomain:
  """
  Retrieve the public (i.e. non-encrypted) domain for that entry.
  - Note: If the entry is private-only, nothing is returned.
  + Note: Even if the entry is private-only, an empty :py:class:`ccf.ledger.PublicDomain` object is returned.
  :return: :py:class:`ccf.ledger.PublicDomain`
  """
  - if self._public_domain == None:
  + if self._public_domain is None:
  buffer = io.BytesIO(_byte_read_safe(self._file, self._public_domain_size))
  self._public_domain = PublicDomain(buffer)
  return self._public_domain
  