Part: modernize C++11

* use nullptr

src/Mod/Part/App/Geometry2d.cpp

@@ -2296,7 +2296,7 @@ PyObject *Geom2dTrimmedCurve::getPyObject(void)
     }
 
     PyErr_SetString(PyExc_RuntimeError, "Unknown curve type");
-    return 0;
+    return nullptr;
 }
 
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