fix(teleport): targetAnchor should also be removed when unmounted

[leaon4]

see test case

[underfin]

Here should check the target existence, because it only insert when has vaild target.

[leaon4]

Thank you, you're right.
Because of your suggestion, I reconsidered four situations of Teleport's props, , and then found more.
h(teleport, props, h('div'))

1. props = {to: null, disabled: false} // illegal
2. props = {to: null, disabled: true} // need to check target
3. props = {to: target, disabled: false} // should remove targetAnchor but not
4. props = {to: target, disabled: true} // should remove targetAnchor but not. Because when disabeld, it will not enter the method of remove.
   https://github.com/vuejs/vue-next/blob/0a6105f8ce4fb6f91947fb9764aae8eec1aded10/packages/runtime-core/src/renderer.ts#L2040-L2046
   So I rewrite it.

[underfin]

Good work :)

[LinusBorg]

@underfin is this ready to be approved?