Only return Fields, Types for Services with Schema

[pkqk]

Unreachable services haven't loaded a schema yet so the definition is nil, causing the meta plugins query to fail with a nil dereference panic.

[nmaquet]

I think it would be slightly nicer to add

if schema == nil {
    return nil
}

to both getTypes and getFields instead of changing the Services method? It's of course equivalent but it will avoid others making the same mistake in the future and keeps the loop in Services() tidier.