FIX: Don't append when there is nothing to append.

app/assets/javascripts/discourse/lib/autocomplete.js.es6

@@ -130,12 +130,15 @@ export default function(options) {
         if (options.transformComplete) {
           term = options.transformComplete(term);
         }
+
+        if (term) {
           var text = me.val();
           text = text.substring(0, completeStart) + (options.key || "") + term + ' ' + text.substring(completeEnd + 1, text.length);
           me.val(text);
           Discourse.Utilities.setCaretPosition(me[0], completeStart + 1 + term.length);
         }
       }
+    }
     closeAutocomplete();
   };