[Flight] Don't block on debug channel if it's not wired up (#33757)

React Elements reference debug data (their stack and owner) in the debug
channel. If the debug channel isn't wired up this can block the client
from resolving.

We can infer that if there's no debug channel wired up and the reference
wasn't emitted before the element, then it's probably because it's in
the debug channel. So we can skip it.

This should also apply to debug chunks but they're not yet blocking
until #33665 lands.