Part: modernize C++11

* use nullptr

src/Mod/Part/App/GeomPlate/PointConstraintPyImp.cpp

@@ -249,7 +249,7 @@ PyObject* PointConstraintPy::pnt2dOnSurf(PyObject *args)
 
 PyObject *PointConstraintPy::getCustomAttributes(const char* /*attr*/) const
 {
-    return 0;
+    return nullptr;
 }
 
 int PointConstraintPy::setCustomAttributes(const char* /*attr*/, PyObject* /*obj*/)