Convert some 0 to nullptr

Change-Id: Ib3ca8e2cc0d63fd35582c73b3cf7c915fb64ddfb
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/modelinglib/qmt/diagram_controller/diagramcontroller.cpp

@@ -326,8 +326,8 @@ private:
 
 DiagramController::DiagramController(QObject *parent)
     : QObject(parent),
-      m_modelController(0),
-      m_undoController(0)
+      m_modelController(nullptr),
+      m_undoController(nullptr)
 {
 }