Part: modernize C++11

* use nullptr

src/Mod/Part/App/GeometryIntExtensionPyImp.cpp

@@ -97,7 +97,7 @@ void GeometryIntExtensionPy::setValue(Py::Long value)
 
 PyObject *GeometryIntExtensionPy::getCustomAttributes(const char* /*attr*/) const
 {
-    return 0;
+    return nullptr;
 }
 
 int GeometryIntExtensionPy::setCustomAttributes(const char* /*attr*/, PyObject* /*obj*/)