### What?
When there's a parallel route adjacent to a tree that has no page
component, it's treated as an invalid entry in `handleAppPing` during
dev HMR, which causes an infinite refresh cycle
### Why?
In #51413, an update was made to `next-app-loader` to support layout
files in parallel routes. Part of this change updated the parallel
segment matching logic to mark the parallel page entry as `[
'@parallel', [ 'page$' ] ]` rather than `[ '@parallel', 'page$' ]`.
This resulted in `handleAppPing` looking for the corresponding page
entry at `client@app@/@parallel/page$/page` (note the `PAGE_SEGMENT`
marker) rather than `client@app@/@parallel/page`, causing the path to be
marked invalid on HMR pings, and triggering an endless fastRefresh.
### How?
A simple patch to fix this would fix this is to update `getEntryKey` to
replace any `PAGE_SEGMENT`'s that leak into the entry which I did in
59a972f53339cf6e444e3bf5be45bf115a24c31a.
The other option that's currently implemented here is to only insert
PAGE_SEGMENT as an array in the scenario where there isn't a page
adjacent to the parallel segment. This is to ensure that the
`parallelKey` is `children` rather than the `@parallel` slot when in
[`createSubtreePropsFromSegmentPath`](59a972f533/packages/next/src/build/webpack/loaders/next-app-loader.ts (L298)).
This seems to not cause any regressions with the issue being fixed in
51413, and also solves this case, but I'm just not 100% sure if this
might break another scenario that I'm not thinking of.
Closes NEXT-1337
Fixes#51951
0b87ba29c4