Fix magn-5555: proxy custom node causes the whole graph evaluates to null

This pull request is to fix defect [MAGN-5555: [user issue]if custom node returns a function pointer then graph returns null](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5555).

That's because for custom node instances in this defect, their custom node definition is just a proxy (custom node definition is a proxy if Dynamo is loading a custom node instance but can't find the corresponding .dyf file, Dynamo still creates a definition but this definition is invalid -- ``CustomNodeDefinition.IsProxy`` returns true). So an exception is thrown out when accessing some properties from the definition during AST compilation. We should compile a custom node instance to null expression if its definition is proxy.  

- [x] @pboyer please help to review the change, thanks!