fix panic in highlighting code from nil Operation

Created by: camdencheek

highlightOp was never initialized following https://github.com/sourcegraph/sourcegraph/pull/34456/files. I'm not sure if this is the best way to do this, but main is currently broken, so I'll leave it to others to clean it up if they have a better pattern for this.

Test plan

Tested locally that panic does not occur.