fix(runtime-core): non-stable Fragment always need to diff its children

[HcySunYang]

close: #2444

[unbyte]

I'm doubt about if the associated issue is a bug, because there is a problem with the code itself.

const block = Vue.ref({id:1});
// ...
<my-test v-for="id in [block.id]" :key="id" />

when block.value is null, [block.value.id] should not work.

---

a more appropriate sample works well: https://codepen.io/unbyte/pen/yLJaMVj?editors=1111

[HcySunYang]

Consider this: https://codepen.io/hcysunyang/pen/LYZRWXp?editors=1111