Transition with mode="in-out" does not remove old element when no other element to transition "in" is present

[rashfael]

Version

3.0.5

Reproduction link

https://jsfiddle.net/5j9hmL70/

Steps to reproduce

1. Press button "TOGGLE"

What is expected?

Text "toggled" should disappear.

What is actually happening?

Text "toggled" does not disappear.

---

I found this problem in a more complex constellation with vue-router, where the optional component does not disappear:

<router-view name="document" v-slot="{ Component }">
  <transition name="tri-view-document" mode="in-out">
    <component v-if="activeDocument" :is="Component" :key="activeDocument.id"/>
  </transition>
</router-view>

[HcySunYang]

When toggling branches with in-out mode, the branch being entered maybe a comment node, which makes the transition invalid and never completes. this PR checks the type of node being entered.