runtime/internal/rt: fix legacy config flow in initMgmt
runtime/internal/rt: fix legacy config flow in initMgmt

We should not return nil at the end of the legacy code path, since that
undoes the fallthrough for legacyConfig != nil

Change-Id: Icebed8b20fd58556e47703d9c76c77e9a25038e0

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref