DYN-7082 Update List.cs => IndexOf

[denizmaral]

Purpose

The changes from #15205 require an update here. If the element is not in the list it should return null instead of -1.

https://jira.autodesk.com/browse/DYN-7082

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.
• This PR modifies some build requirements and the readme is updated
• This PR contains no files larger than 50 MB

Release Notes

DYN-7082 Update List.cs => IndexOf

Reviewers

(FILL ME IN) Reviewer 1 (If possible, assign the Reviewer for the PR)

(FILL ME IN, optional) Any additional notes to reviewers or testers.

FYIs

(FILL ME IN, Optional) Names of anyone else you wish to be notified of

[github-actions]

See the ticket for this pull request: https://jira.autodesk.com/browse/DYN-7082

[QilongTang]

@denizmaral Dynamo team appreciate your contribution, way to go!