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
3fc65bac75