No need to query the API about nothing

extensions/mentions/js/src/composer-autocomplete.js

@@ -152,6 +152,7 @@ export default function() {
           dropdown.active(true);
 
           clearTimeout(searchTimeout);
+          if (typed) {
             searchTimeout = setTimeout(function() {
               var typedLower = typed.toLowerCase();
               if (searched.indexOf(typedLower) === -1) {
@@ -162,6 +163,7 @@ export default function() {
               }
             }, 250);
           }
+        }
       });
   });
 }