update test case

tests/popover.test.js

@@ -21,7 +21,7 @@ describe('Popover', function() {
     );
 
     const popup = popover.getPopupDomNode();
-    expect(popup).not.toBe(undefined);
+    expect(popup).not.toBe(null);
     expect(popup.className).toContain('ant-popover-placement-top');
     expect(popup.innerHTML).toMatch(/<div class="ant-popover-title".*?>code<\/div>/);
     expect(popup.innerHTML).toMatch(/<div class="ant-popover-inner-content".*?>console\.log\('hello world'\)<\/div>/);