Skip empty prefetch request for dynamic routes (#78436)
In the Segment Cache implementation of prefetching, a prefetch happens
in two phases: first we fetch the route tree, then we fetch the data
itself.
The route tree prefetch tells us important metadata about the structure
of the target route, like whether it shares any layouts with the current
page. We can use this information to omit parts of the tree from
subsequent data request.
In this PR, I've added a new piece of metadata to the route tree
prefetch response: `hasLoadingBoundary`. For each segment in the tree,
this represents whether there is a `loading.tsx` boundary somewhere
within it. This is useful because we know that during a prefetch, the
server will only render up to the nearest loading boundary (unless
prefetch={true} is set); if there are no loading boundaries, then the
server will return an empty response.
In the latter case, we can use this information to skip the data
prefetch entirely, freeing up bandwidth for other requests in the queue.
