app router: ensure static prefetch renders loading.js (#55950)
Should fix the problem surfaced in https://github.com/vercel/next.js/issues/43548#issuecomment-1732416492
The scenario in which this breaks is when we're asking the app renderer to generate a prefetch payload statically. The renderer was not checking if there were loading.js below the current rendering level, which is the root when statically rendering, so it would always render null, which was incorrect.
Tested out with the original repro https://next-instant-loading-8nv6psqod-feedthejim.vercel.app
