[applets/systemtray] Default value for ItemLoader.source

Summary: When none of the valid conditions are met, function should return default value for ItemLoader source. If not, error is printed in logs.

Test Plan: No warning about undefined value for url

Reviewers: #plasma_workspaces, #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28259

applets/systemtray/package/contents/ui/items/ItemLoader.qml

@@ -30,5 +30,6 @@ Loader {
         } else if (model.itemType === "StatusNotifier") {
             return "StatusNotifierItem.qml"
         }
+        return ""
     }
 }