v23/rpc/reflect_invoker: Make error message for empty struct object more helpful.
v23/rpc/reflect_invoker: Make error message for empty struct object more
helpful.

Ali informed me that the error message return by
NewServer(ctx, name, struct{}{}, ...) is not very descriptive.
Specifically:
type struct {} has no compatible methods: map[]

This change makes the error more helpful:
type struct {} has no compatible methods: no methods on object.

Change-Id: I4b5fdbc0238f49b1602467a14680b1b120b64005

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23