Fix magn5233: List.map returns function obejct in some caseseven when the function is completed

Fix defect [MAGN 5233: List.map returns function obejct in some caseseven when the function is completed] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5233)

If some inputs of a node have default values, when the node is compiled to AST, we should use these default values. I deleted ``NodeModel.GetConnectedInputs()`` because it is only used in one place (and I cannot find out a better name... ``GetIndexOfInputThatHasInput()``?!). 

Reviewer:
- [x] @Steell