Skip empty requests for cross-signing keys when user only has one device.

Cause: Request button pressed in user settings.

Result: Request body is jsNull; sendToDevice requires jsObject.

src/encryption/Olm.cpp

@@ -1613,6 +1613,9 @@ request_cross_signing_keys()
                 body[local_user][dev] = secretRequest;
         }
 
+        if (body.empty())
+            return;
+
         http::client()->send_to_device<mtx::events::msg::SecretRequest>(
           http::client()->generate_txn_id(),
           body,