Fix MAGN 4105 Dynamo crash on Offsetting perimeter curves

This crash has nothing to do with geometry. 

The crash is because of this comparison: ``value == null`` where the declaration type of ``value`` is ``dynamic``.  If the real type of ``value`` is ``StackValue`` which doesn't override operator ``==``, an exception RuntimeBinderException will be thrown out. 

So replacing ``==`` with ``Object.ReferenceEquals()`` (why not using``Object.Equals()``, or ``value.Equals(null)``? See [this StackOverflow question](http://stackoverflow.com/questions/3507383/equalsitem-null-or-item-null) )