compose: Ensure we don't warn unresolved topic if not composing for a topic.

In future commits, it will become possible to have a non-null
`compose_state.topic()` while in private message view, because
we'll be keeping that state for switching between the stream
and private message views. See #21853 for further context.
We don't want to warn about a resolved topic unless the topic
is actively visible in the compose box.

static/js/compose_validate.js

@@ -170,6 +170,9 @@ export function clear_topic_resolved_warning() {
 }
 
 export function warn_if_topic_resolved(topic_changed) {
+    if (compose_state.recipient_has_topics()) {
+        return;
+    }
     // This function is called with topic_changed=false on every
     // keypress when typing a message, so it should not do anything
     // expensive in that case.