WelcomePage: Fixing missing images and warnings

The property help source was always defined.

The actual reason for the warnings was that we did not check
if model.imageUrl is undefined and java script turns an undefined
value into the string "undefined".

Change-Id: Ifb433ff5cdc3142ff5e552ff1b01b04b1a859590
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

share/qtcreator/welcomescreen/widgets/CustomizedGridView.qml

@@ -52,16 +52,9 @@ GridView {
             property string sourcePrefix: isHelpImage ? "image://helpimage/" : ""
 
             property string mockupSource: model.imageSource
-            property string helpSource: model.imageUrl !== "" ? sourcePrefix + encodeURI(model.imageUrl) : ""
-
-            imageSource: getImageSource()
-
-            function getImageSource() {
-                var value = delegate.isVideo ? "" : (model.imageSource === undefined ? delegate.helpSource : delegate.mockupSource)
-                if (value === undefined)
-                    value = "";
-            }
+            property string helpSource: (model.imageUrl !== "" && model.imageUrl !== undefined) ? sourcePrefix + encodeURI(model.imageUrl) : ""
 
+            imageSource: isVideo ? "" : (model.imageSource === undefined ? delegate.helpSource : delegate.mockupSource)
 
             videoSource: isVideo ? (model.imageSource === undefined ? model.imageUrl : mockupSource) : ""