Allow missing CacheNodeSeedData during prefetch (#72857)
Updates the client PPR implementation to gracefully handle missing
CacheNodeSeedData when constructing a prerender tree. Currently this
never happens because we always block until we receive a server
response. However, in the new Segment Cache, we will not bother to wait
if a URL has not already been prefetched, since it's usually faster to
go straight to the dynamic request.
