Avoid kw::Empty when dealing with rustc_allowed_through_unstable_modules.

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.

compiler/rustc_error_codes/src/error_codes/E0789.md

@@ -14,7 +14,7 @@ Erroneous code example:
 
 #![unstable(feature = "foo_module", reason = "...", issue = "123")]
 
-#[rustc_allowed_through_unstable_modules]
+#[rustc_allowed_through_unstable_modules = "deprecation message"]
 // #[stable(feature = "foo", since = "1.0")]
 struct Foo;
 // ^^^ error: `rustc_allowed_through_unstable_modules` attribute must be