Change $select.clear() so it sets value to null instead of undefined

src/uiSelectController.js

@@ -451,7 +451,7 @@ uis.controller('uiSelectCtrl',
   };
 
   ctrl.clear = function($event) {
-    ctrl.select(undefined);
+    ctrl.select(null);
     $event.stopPropagation();
     $timeout(function() {
       ctrl.focusser[0].focus();

test/select.spec.js

@@ -552,7 +552,7 @@ describe('ui-select tests', function() {
 
     // Trigger clear.
     el.find('.select2-search-choice-close').click();
-    expect(scope.selection.selected).toEqual(undefined);
+    expect(scope.selection.selected).toEqual(null);
 
     // If there is no selection it the X icon should be gone.
     expect(el.find('.select2-search-choice-close').length).toEqual(0);