Better fix for INC-140

Created by: olafurpg

The previous fix didn't address the root regression from the PR https://github.com/sourcegraph/sourcegraph/pull/40610

The value of occurrences went from undefined to [], causing the if condition to fail where it used to pass because [] is a truthy value in JavaScript.

Test plan

See the CI go green. I manually validated that syntax highlighting is still working as expected, and I'm still investigating why the integration tests didn't catch the original regression.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.