fix(runtime-core): normalize empty extra props to null for cloneVnode

[underfin]

fix #1704

This pr fix empty extra props, this will change patchFlag(this can fix the issue).
But the real reason with this issue is here.
https://github.com/vuejs/vue-next/blob/5c74243211495e33319218584993177c9d2e2c80/packages/runtime-core/src/componentRenderUtils.ts#L250-L252
The pushed vnode will cause error when patch this(there are block node and previous block tree maybe hasn't this node)

[yyx990803]

The root cause of #1704 is because v-if in off state renders a comment node, which is then mistakenly filtered out as a user comment when trying to determine fallthrough target in a fragment root with user comments. See 355c052

This fix should be unnecessary since in compiled output cloneVNode should only happen for real root nodes which are always blocks themselves.

[sheldonstewart]

so what is the solution?