fix(go): resolve custom base_url for empty default region (#15043)

### What problem does this PR solve?

Fixes custom `base_url` resolution when a model instance has no
configured region.

Some drivers read custom base URLs from `BaseURL[""]` when
`apiConfig.Region` is empty, while others normalize empty region to
`"default"` and read `BaseURL["default"]`. This PR adds the `"default"`
alias only for empty-region custom base URLs while preserving the
existing empty-region key.

Closes #15042

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)