message: remove redundant check and add comment
diff: function memberEventDiff(ev) {
  const diff = {
  - // a Member Event is a State Event and so its State Key must not be undefined.
  - isMemberEvent: ev.getType() === 'm.room.member' && ev.getStateKey() !== undefined,
  + isMemberEvent: ev.getType() === 'm.room.member',
  };
  + // If is not a Member Event then the other checks do not apply, so bail early.
  if (!diff.isMemberEvent) return diff;
  const content = ev.getContent();
  