PD: modernize C++11

* use nullptr

src/Mod/PartDesign/App/AppPartDesign.cpp

@@ -72,7 +72,7 @@ PyMOD_INIT_FUNC(_PartDesign)
     }
     catch(const Base::Exception& e) {
         PyErr_SetString(PyExc_ImportError, e.what());
-        PyMOD_Return(0);
+        PyMOD_Return(nullptr);
     }
 
     PyObject* mod = PartDesign::initModule();