Fixed issue with loading "DSVarArgFunction" node (Definition == null)

There's a problem with loading ```DSVarArgFunction``` node type by following the steps below:

- Launch Dynamo and create a ```List.Create``` node
- Press the ```+``` sign to add few input ports
- Save the file, and open it again in Dynamo
- Note that the input ports are gone

This is because ```DSVarArgFunction``` node was created when ```Activator``` calls its default constructor (without the argument of ```FunctionDescriptor```). This fix removes the default constructor of ```DSVarArgFunction```, making sure it will never be called by anyone (since ```DSVarArgFunction``` class refers to its ```Definition``` property everywhere and it is not quite ready to deal with situation when it has no descriptor).