DYN-2273 - Fix crash by compiling null input to null AST node

[aparajit-pratap]

Purpose

JIRA: https://jira.autodesk.com/browse/DYN-2273

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning, and are documented in the API Changes document.

Reviewers

@DynamoDS/dynamo

[QilongTang]

@aparajit-pratap , my memory fading. Can you describe null node a bit?

[aparajit-pratap]

@QilongTang by nullnode, I meant Null AST node, sorry for not making that clear before. We have a special AST node to represent null. In the case where an input AST is null, we would rather propagate the null than throw an exception either at the time of AST compilation or at runtime, which was what was happening before.

[QilongTang]

@aparajit-pratap Thank you, makes sense. LGTM