Don't warn empty branches unreachable for now
The [stabilization](https://github.com/rust-lang/rust/pull/122792) of `min_exhaustive_patterns` updated the `unreachable_pattern` lint to trigger on empty arms too. This has caused some amount of churn, and imposes an unjoyful `#[allow(unreachable_patterns)]` onto library authors who want to stay backwards-compatible.
While I think the lint should eventually cover these cases, for transition's sake I'd prefer to revert linting to what it was prior to stabilization, at least for now.
Fixes https://github.com/rust-lang/rust/issues/129031.
r? ``@compiler-errors``
Jubilee
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