Another option here would be to have two separate getters, as it's really two separate use cases.

One that directly returns the value stored in git config
One that applies a default value if the value stored in git config is nil

The first is used when describing a given hub's configuration.

The second is treated as the active value for a github config when used within the context of a zenflow-configured project.  This way, you can over-ride the api base url value, but, for existing zenflow users using github.com, you don't have to explicitly configure the value, DEFAULT_API_BASE_URL gets used automatically.