FIX: ESC to dismiss empty draft topic composer

Previously originalTitle was set to undefined leading to
titleDirty being unconditionally undefined for new topics

That mean you could not dismiss the composer via ESC

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

@@ -785,11 +785,14 @@ const Composer = RestModel.extend({
       });
     }
 
+    this.set("originalTitle", "");
+
     if (opts.title) {
       this.set("title", opts.title);
     }
 
     this.set("originalText", opts.draft ? "" : this.reply);
+
     if (this.editingFirstPost) {
       this.set("originalTitle", this.title);
     }