QmlProfiler: Don't use the model factory if it doesn't exist

The model factory only exists if there is a plugin providing it. Also,
it's a good idea to initialize it.

Change-Id: I904bae1e733c37699a99e84e79c67ab4e8aea805
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>

src/plugins/qmlprofiler/qmlprofilerplugin.h

@@ -46,7 +46,7 @@ class QmlProfilerPlugin : public ExtensionSystem::IPlugin
     Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QtCreatorPlugin" FILE "QmlProfiler.json")
 
 public:
-    QmlProfilerPlugin() {}
+    QmlProfilerPlugin() : factory(0) {}
 
     bool initialize(const QStringList &arguments, QString *errorString);
     void extensionsInitialized();