Conditional children break key handling in wrapped transitions

[CyberAP]

Version

3.0.0-alpha.9

Reproduction link

https://codesandbox.io/s/strange-cache-ert3j

Steps to reproduce

1. Create a wrapped transition (transition component as root and a slot)
2. Use that transition on a single child component with a structural directive (v-if)

What is expected?

Should produce no warnings

What is actually happening?

A warning is displayed:

[Vue warn]: <TransitionGroup> children must be keyed. 
  at <Anonymous tag="div" name="transition-test" > "" 
  at <TransitionComponent> "" 
  at <App> "" 
  at <Anonymous> (Root)