fix: node.insertBefore parameter issue

[gaokun]

Root Cause

According to insertBefore API doc:

Note: referenceNode is not an optional parameter. You must explicitly pass a Node or null.

so we have to pass null, if referenceNode is undefined for 2nd parameter.

References:

1. insertBefore equals appendChild when referenceNode is null, refer to Vue source code

2. In Chrome, Firefox, Safari, it works when referenceNode is undefined, the same as null dose.

But happy-dom doesn't, it is strict as API said. check this code

3. An issue in ant-design-vue was caused by this.