This is a stepping-stone PR for the provider source project. In this PR
"legcay-stype" FQNs are created from the provider name string. Future
work involves encoding the FQN directly in the AbsProviderConfig and
removing the calls to addrs.NewLegacyProvider().
* command/providers schemas: return empty json object if config parses successfully but no providers found
* command/show (state): return an empty object if state is nil