Revise null-ability of request/response in IPC

  - Client/Server: both request and response cannot be null

  - MessageServer/MessageClient: message name cannot be null,
    but Variant payload can be null in request (function call without
    argument) as well as in response (void function).

  - Example updated.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>