Bugfix: Do not require release notes for Firebase releases

[priitlatt]

Not all Firebase App Distribution releases have release notes attached to them. This causes related firebase-app-distribution actions to fail with the following exception:

Traceback (most recent call last):
  File "/Users/builder/.pyenv/versions/3.8.13/lib/python3.8/site-packages/codemagic/cli/cli_app.py", line 212, in invoke_cli
    CliApp._running_app._invoke_action(args)
  File "/Users/builder/.pyenv/versions/3.8.13/lib/python3.8/site-packages/codemagic/cli/cli_app.py", line 168, in _invoke_action
    return cli_action(**action_args)
  File "/Users/builder/.pyenv/versions/3.8.13/lib/python3.8/site-packages/codemagic/cli/cli_app.py", line 463, in wrapper
    return func(self, *args, **kwargs)
  File "/Users/builder/.pyenv/versions/3.8.13/lib/python3.8/site-packages/codemagic/tools/firebase_app_distribution/actions/get_latest_build_version_action.py", line 23, in get_latest_build_version
    releases = self.client.releases.list(app_identifier, limit=1)
  File "/Users/builder/.pyenv/versions/3.8.13/lib/python3.8/site-packages/codemagic/google/resource_managers/mixins/listing_manager_mixin.py", line 68, in list
    resources.extend(self.resource_type(**item) for item in response[self.resource_type.get_label()])
  File "/Users/builder/.pyenv/versions/3.8.13/lib/python3.8/site-packages/codemagic/google/resource_managers/mixins/listing_manager_mixin.py", line 68, in <genexpr>
    resources.extend(self.resource_type(**item) for item in response[self.resource_type.get_label()])
TypeError: __init__() missing 1 required positional argument: 'releaseNotes'

Make releaseNotes field on Release object optional.