Polish tests around JSON.parse and possible null values in historical reviewer tracking

The following discussion from !187457 (merged) should be addressed:

• @arfedoro started a discussion:

  suggestion (non-blocking) 🏓

  Just in case of JSON.parse(list) failing (it can happen when parsing), what do you think about wrapping this part in try/catch block:

  try {
    return list ? JSON.parse(list) : [];
  } catch {
    return [];
  }