wip narrow: getNarrowFromMessage never a stream narrow

There's no reason we would sometimes want a stream narrow here.
This is just how it was when this function was first written, for
narrowing to a message's conversation, in 34ab1f9 (zulip#333) in 2017.

This case also should be impossible anyway, because message topics
should always be nonempty -- we convert empty to "(no topic)".

src/utils/__tests__/narrow-test.js

@@ -327,16 +327,6 @@ describe('getNarrowFromMessage', () => {
     expect(actualNarrow).toEqual(expectedNarrow);
   });
 
-  test('for stream message with empty topic, returns a stream narrow', () => {
-    // TODO this behavior seems pretty dubious
-    const message = eg.streamMessage({ subject: '' });
-    const expectedNarrow = streamNarrow(eg.stream.name);
-
-    const actualNarrow = getNarrowFromMessage(message, eg.selfUser);
-
-    expect(actualNarrow).toEqual(expectedNarrow);
-  });
-
   test('for stream message with nonempty topic, returns a topic narrow', () => {
     const message = eg.streamMessage();
     const expectedNarrow = topicNarrow(eg.stream.name, message.subject);

src/utils/narrow.js

@@ -329,9 +329,6 @@ export const getNarrowFromMessage = (message: Message | Outbox, ownUser: User) =
   } else {
     const streamName = streamNameOfStreamMessage(message);
     const topic = message.subject;
-    if (topic && topic.length) {
-      return topicNarrow(streamName, topic);
-    }
-    return streamNarrow(streamName);
+    return topicNarrow(streamName, topic);
   }
 };