GCP Application Default Credentials config throws Exception

[spearl]

If using GCP Application Default Credentials, such as in that section in the README, and you have your user config like the example e.g. user: {auth-provider: {name: gcp}} then a NoMethodError on line 177 is thrown.

This happens because there is no config in the auth provider so the first argument to expand_command_option is nil.

The workaround is simply to set an empty config (config: {}) in the auth-provider, but this method needs to be fixed to avoid throwing an exception.