When OIDC_* environment variables were set but no oidc.enc config file
existed, cfg.OIDC was nil and MergeFromEnv would silently return without
applying the env vars (due to nil receiver check).
Fix: Initialize cfg.OIDC to default values before merging env vars if
it's nil. This ensures OIDC can be configured purely through environment
variables without requiring a pre-existing config file.
Related to #853
rcourtman
9dc365c9c6