Path: modernize C++11

* use nullptr

src/Mod/Path/App/AppPath.cpp

@@ -65,7 +65,7 @@ PyMOD_INIT_FUNC(Path)
     }
     catch(const Base::Exception& e) {
         PyErr_SetString(PyExc_ImportError, e.what());
-        PyMOD_Return(NULL);
+        PyMOD_Return(nullptr);
     }
 
     PyObject* pathModule = Path::initModule();