FIX: ensures we don't attempt to create a new PM on an existing topic (#9029)

This fix attempts to both fix it at UI level and server side. A previous attempt related to this behavior has been made in commit: https://github.com/discourse/discourse/commit/49c750ca7890490b1405f940b1eb2e7a4f3f69f6

app/assets/javascripts/discourse/models/composer.js.es6

@@ -912,7 +912,7 @@ const Composer = RestModel.extend({
   },
 
   createPost(opts) {
-    if (this.action === CREATE_TOPIC) {
+    if (CREATE_TOPIC === this.action || PRIVATE_MESSAGE === this.action) {
       this.set("topic", null);
     }