[Segment Cache] Respond with 204 on cache miss (#73649)
Currently, when the client prefetches a segment, the server responds
with a 404 if it cannot fulfill the request. This updates it to respond
with a 204 No Content instead, since it's not an error for the client to
request a segment whose prefetch hasn't been generated.
When responding with 204, the server also sends the 'x-nextjs-postponed'
header, but only if PPR is enabled for the route. The client can use the
absence of this header to distinguish between a regular cache miss and a
miss due to PPR being disabled. In a later PR, I will update the client
to use this information to fall back to the non-PPR behavior.
