move Placement.isNull to Placement.isIdentity

add isNull for backward compatibility

src/Base/PlacementPyImp.cpp

@@ -247,8 +247,20 @@ void PlacementPy::setRotation(Py::Object arg)
     throw Py::TypeError("either Rotation or tuple of four floats expected");
 }
 
-PyObject *PlacementPy::getCustomAttributes(const char* /*attr*/) const
+PyObject *PlacementPy::getCustomAttributes(const char* attr) const
 {
+    // for backward compatibility
+    if (strcmp(attr, "isNull") == 0) {
+        PyObject *w, *res;
+#if PY_MAJOR_VERSION >= 3
+        w = PyUnicode_InternFromString("isIdentity");
+#else
+        w = PyString_InternFromString("isIdentity");
+#endif
+        res = PyObject_GenericGetAttr(const_cast<PlacementPy *>(this), w);
+        Py_XDECREF(w);
+        return res;
+    }
     return 0;
 }