fix(typeahead): return null if empty

- Return `null` instead to unify falsy values

Closes #4078
Fixes #3176

Author: wesleycho

src/typeahead/test/typeahead.spec.js

@@ -258,7 +258,7 @@ describe('typeahead tests', function () {
       expect($scope.result).toEqual(undefined);
       expect($scope.form.input.$error.editable).toBeTruthy();
       changeInputValueTo(element, '');
-      expect($scope.result).toEqual('');
+      expect($scope.result).toEqual(null);
       expect($scope.form.input.$error.editable).toBeFalsy();
     });
 

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;