[Breaking] Skip Next.js internal routes `/_next` in middleware (#84239)
### Why?
When the user hasn't set a matcher in the middleware, it always hits the
internal route `/_next/`. This is redundant, though quite common, as it
serves all the static assets. Therefore, skip the internal route by
default when no matcher is set.
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>