Handle empty/incomplete gcloud config file gracefully

[mattwynne]

Summary

When ~/.config/gcloud/configurations/config_default exists but is empty or missing the required [core] section (e.g. after running gcloud auth list on a fresh install), get_configuration_data/1 crashes trying to access keys on a nil/empty map. This crashes the GenServer with a CaseClauseError and prevents any fallback credential sources from being tried.

This change makes get_configuration_data/1 pattern-match for the expected structure and return nil when it's not present, with a warning log that tells the user how to fix it. This lets the credential fallback chain in from_gcloud_adc/1 continue to metadata or other sources.

Before

** (CaseClauseError) no case clause matching: :error
    (goth 1.4.5) lib/goth/token.ex:252: Goth.Token.request/1

After

[warning] Gcloud configuration file ... is missing required [core] section
with project and account settings. Run 'gcloud init' or delete this file to resolve.

And Goth continues to try other credential sources.

Licensing:
This contribution is made by employees of Mechanical Orchard, Inc. under the terms of the project's license.