Fix: Avoid null error in toc controller

[JoshDevHub]

Because

Currently there's a null error that can happen in the toc stimulus controller if a lesson starts with a comment.

This PR

• Removes the possibility for this error by using optional chaining and an early return

Issue

Closes #4434

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the keyword: brief description of change format, using one of the following keywords:
  • Feature - adds new or amends existing user-facing behavior
  • Chore - changes that have no user-facing value, refactors, dependency bumps, etc
  • Fix - bug fixes
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] I have verified all tests and linters pass after making these changes.
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [ ] If applicable, this PR includes new or updated automated tests

[JoshDevHub]

To QA in the review app:

1. Visit a lesson that has a linter comment. An example would be lessons/foundations-installing-node-js
2. Open the console and verify no null error.