TD: modernize C++11

* use nullptr

src/Mod/TechDraw/App/DrawViewSymbolPyImp.cpp

@@ -71,7 +71,7 @@ PyObject* DrawViewSymbolPy::dumpSymbol(PyObject *args)
 
 PyObject *DrawViewSymbolPy::getCustomAttributes(const char* /*attr*/) const
 {
-    return 0;
+    return nullptr;
 }
 
 int DrawViewSymbolPy::setCustomAttributes(const char* /*attr*/, PyObject* /*obj*/)