fix(typeahead): return null if empty

- Return `null` instead to unify falsy values

Author: wesleycho

src/typeahead/typeahead.js

@@ -290,7 +290,7 @@ angular.module('ui.bootstrap.typeahead', ['ui.bootstrap.position', 'ui.bootstrap
           if (!inputValue) {
             // Reset in case user had typed something previously.
             modelCtrl.$setValidity('editable', true);
-            return inputValue;
+            return null;
           } else {
             modelCtrl.$setValidity('editable', false);
             return undefined;