In #61573, I updated the navigation reducer to request a new prefetch
entry if it's stale. But this has the unintended consequence of making
instant loading states effectively useless after 30s (when the prefetch
would have expired). Blocking navigation and then rendering the loading
state isn't ideal - if we have some loading data in a cache node, we
should re-use it.

Now that #62346 stores loading data in the `CacheNode`, we can copy over
`loading` during a navigation.

This PR repurposes `fillCacheWithDataProperty` which wasn't being used
anywhere, to instead be a utility we can use to programmatically trigger
a lazy fetch on a particular segment path by nulling out it's data while
copying over other properties. We could have used the existing util
as-is, but ideally we only have a single spot where lazy fetching can
happen, which currently is in `LayoutRouter`.

When a stale prefetch entry is detected, rather than applying the data
to the tree, this PR will copy over the `loading` nodes and will
"delete" the data so it can be refetched.

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Closes NEXT-2806

This ensures that even if a `loading.js` returns `null`, that we still
render a `Suspense` boundary, as it's perfectly valid to have an empty
fallback.

This was accidentally lost in #62346 -- this brings back the
`hasLoading` prop which will check the loading module itself (rather
than the `ReactNode`) for truthiness, and I've added a test to avoid
another regression.

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Closes NEXT-2936