message: Simplify handling optional string result in agent api.
diff: @@ -92,9 +92,9 @@ pub extern fn indy_parse_msg(command_handle: i32,
  Box::new(move |result| {
  let (err, sender_vk, msg) = result_to_err_code_2!(result, None, Vec::new());
  let (msg_data, msg_len) = vec_to_pointer(&msg);
  - let c_sender_vk = sender_vk.clone().map(CStringUtils::string_to_cstring).unwrap_or(CStringUtils::string_to_cstring(String::new()));
  + let sender_vk = sender_vk.map(CStringUtils::string_to_cstring);
  cb(command_handle, err,
  - if sender_vk.is_some() { c_sender_vk.as_ptr() } else { ptr::null() },
  + sender_vk.as_ref().map(|vk| vk.as_ptr()).unwrap_or(ptr::null()),
  msg_data, msg_len)
  })
  )));
  