### What
When navigating to a page that uses a loading boundary + parallel route,
an empty white screen would be displayed rather than the loading state /
final state
### Why
With parallel routes, the RSC data is an array of data paths, each
corresponding with one of the parallel segments rendered on the page.
During the navigation event, when we iterate over this data, we call
`applyFlightData` with this data path & an empty cache node.
`applyFlightData` checks to see if the flight data contains cache nodes
("seed data"). If it doesn't, then that means it has no work to do, and
it bails out. Pre-PPR and in the case of having a `loading.js` file,
`walkTreeWithFlightRouterState` doesn't return any seed data, just
router state. This means that `applyFlightData` will not have any work
to do on the new cache node, and leaves it untouched.
Once `applyFlightData` is finished, but while still in the flight data
path loop, we reassign `currentCache` to the empty cache object we
created prior to `applyFlightData`. But since that cache node has
remained empty, the next iteration of the loop is going to be inspecting
a now empty cache, rather than the actual "current" cache. Now there's
no existing cache to copy into the new cache. The app now doesn't know
about any cache nodes.
### How
It doesn't seem like we should be re-assigning `currentCache` to the new
cache. In the context of a navigation, it seems more accurate to always
assume `currentCache` is the cache _now_, since it won't actually be
applied to the state until the action has finished (`mutable.cache` is
currently taking care of this).
Closes NEXT-2223
Fixes#61080
a4f46bc157