Start: modernize C++11

* use nullptr

src/Mod/Start/Gui/AppStartGui.cpp

@@ -70,7 +70,7 @@ PyMOD_INIT_FUNC(StartGui)
 {
     if (!Gui::Application::Instance) {
         PyErr_SetString(PyExc_ImportError, "Cannot load Gui module in console application.");
-        PyMOD_Return(0);
+        PyMOD_Return(nullptr);
     }
 
     // load dependent module
@@ -79,7 +79,7 @@ PyMOD_INIT_FUNC(StartGui)
     }
     catch(const Base::Exception& e) {
         PyErr_SetString(PyExc_ImportError, e.what());
-        PyMOD_Return(0);
+        PyMOD_Return(nullptr);
     }
     catch (Py::Exception& e) {
         Py::Object o = Py::type(e);

src/Mod/Start/Gui/DlgStartPreferencesImp.h

@@ -34,7 +34,7 @@ namespace StartGui {
         Q_OBJECT
 
     public:
-        DlgStartPreferencesImp( QWidget* parent = 0 );
+        DlgStartPreferencesImp( QWidget* parent = nullptr );
         ~DlgStartPreferencesImp();
 
     protected: