Gui: modernize C++11

* use nullptr

src/Gui/CommandPyImp.cpp

@@ -411,7 +411,7 @@ PyObject* CommandPy::findCustomCommand(PyObject* args)
 
 PyObject *CommandPy::getCustomAttributes(const char* /*attr*/) const
 {
-    return 0;
+    return nullptr;
 }
 
 int CommandPy::setCustomAttributes(const char* /*attr*/, PyObject* /*obj*/)