GCE plugin fails when creating windows hosts if using application default auth

The 'Prepare Windows User' will set the env variable `GOOGLE_APPLICATION_CREDENTIALS` to an empty string when the `molecule_yml.driver.service_account_file`/`GCP_SERVICE_ACCOUNT_FILE` is undefined (which it commonly is when using `auth_kind: application`). windows_auth.py then fails to find the application credentials json file since GOOGLE_APPLICATION_CREDENTIALS is `""`

https://github.com/ansible-community/molecule-plugins/blob/196ed1eb034ebf343a270102179c98e84f49eb71/src/molecule_plugins/gce/playbooks/tasks/create_windows_instance.yml#L64C44-L64C44

Start at src/molecule_plugins/gce/playbooks/tasks/create_windows_instance.yml around line 64, then inspect windows_auth.py and the application-default authentication path. Reproduce a Windows host creation with auth_kind: application and verify that an undefined service account file does not prevent the application credentials JSON from being found.

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