`prefer_void_to_null` false positive for extension types

[lrhn]

Example:

extension type B<T>(T? _) {}
extension type N(Null _) implements B<Never> {}

which yields the info:

Analyzing badwarn.dart...

   info - badwarn.dart:2:18 - Unnecessary use of the type 'Null'.
          Try using 'void' instead. - prefer_void_to_null

1 issue found.

Since changing Null to void would be a compile-time error, extension type representation types might not be the best place to apply that info.
(Any time a type has to be a subtype of some other type, suggesting void instead of Null risks being invalid.)

[eernstg]

With null safety, prefer_void_to_null could be obsolete entirely. Cf. #59309.

[pq]

Fixed w/ d9731ac