The existing code produces `Some(kw::Empty)` for these invalid forms:
- a non-name-value, e.g. `#[rustc_allowed_through_unstable_modules]`
- a non-string arg, e.g. `#[rustc_allowed_through_unstable_modules = 3]`
The new code avoids the `kw::Empty` and is a little shorter. It will
produce `None` in those cases, which means E0789 won't be produced if
the `stable` attribute is missing for these invalid forms. This doesn't
matter, because these invalid forms will trigger an "malformed
`rustc_allowed_through_unstable_modules` attribute" anyway.
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