[Fizz] Make ViewTransition enter/exit/share null the same as none (#33331)

I believe that these mean the same thing. We don't have to emit the
attribute if it's `none` for these cases because if there is no matching
scenario we won't apply the animation in this case.

The only case where we have to emit `none` in the attribute is for
`vt-update` because those can block updates from propagating upwards.