Part: modernize C++11

* use nullptr

src/Mod/Part/App/ArcOfConicPyImp.cpp

@@ -47,7 +47,7 @@ PyObject *ArcOfConicPy::PyMake(struct _typeobject *, PyObject *, PyObject *)  //
     // never create such objects with the constructor
     PyErr_SetString(PyExc_RuntimeError,
         "You cannot create an instance of the abstract class 'ArcOfConic'.");
-    return 0;
+    return nullptr;
 }
 
 // constructor method
@@ -234,7 +234,7 @@ void  ArcOfConicPy::setYAxis(Py::Object arg)
 
 PyObject *ArcOfConicPy::getCustomAttributes(const char* ) const
 {
-    return 0;
+    return nullptr;
 }
 
 int ArcOfConicPy::setCustomAttributes(const char* , PyObject *)