Add missing returns in system channels handlers (#21523)

The Windows and GLFW embeddings both had cases of missing returns after
calling `Error` on a method channel result object, leading to incorrect
double-calls on the result if the error paths were ever hit.

shell/platform/glfw/text_input_plugin.cc

@@ -139,6 +139,7 @@ void TextInputPlugin::HandleMethodCall(
     if (client_config.IsNull()) {
       result->Error(kBadArgumentError,
                     "Could not set client, missing arguments.");
+      return;
     }
     client_id_ = client_id_json.GetInt();
     input_action_ = "";

shell/platform/windows/cursor_handler.cc

@@ -40,6 +40,7 @@ void CursorHandler::HandleMethodCall(
     if (kind_iter == arguments.end()) {
       result->Error("Argument error",
                     "Missing argument while trying to activate system cursor");
+      return;
     }
     const auto& kind = std::get<std::string>(kind_iter->second);
     delegate_->UpdateFlutterCursor(kind);