narrow_state: Disable coverage requirement for unlikely case.

I'm not actually sure if this is possible; it's been copy/pasted from
the channel/topic code path, but even with that one, it seems like the
topic being `undefined` is impossible (that's not how operators work),
and the channel being undefined might be something that should fail in
`Filter` initialization, not here.

web/src/narrow_state.ts

@@ -299,6 +299,7 @@ export function _possible_unread_message_ids(
             return [];
         }
         if (sub === undefined || topic_name === undefined) {
+            /* istanbul ignore next */
             return [];
         }
         return unread.get_msg_ids_for_topic(sub.stream_id, topic_name);