fix: harden lookup of credentials id_token attribute (#434)

Author: crwilcox

google/cloud/firestore_v1/base_client.py

@@ -180,7 +180,10 @@ def _emulator_channel(self, transport):
  # https://github.com/googleapis/python-firestore/issues/359
  # Default the token to a non-empty string, in this case "owner".
  token = "owner"
- if self._credentials is not None and self._credentials.id_token is not None:
+ if (
+ self._credentials is not None
+ and getattr(self._credentials, "id_token", None) is not None
+ ):
  token = self._credentials.id_token
  options = [("Authorization", f"Bearer {token}")]