Do not allow null RPC input in invocation paths

RFC6020 and RFC7950 make it clear the RPC input statement is always present. DOM perspective this means that RPCs always have an input NormalizedNode.

DOMRpcService.invokeRpc() should therefore either not allow nulls in its API contract, or have @implSpec of turning nulls into an empty ContainerNode, on which presence of mandatory leaves is enforced (triggering IllegalArgumentException in such a case).

This change is required to correctly account for RPC input such that default values can be inferred.