NullableTypeDeclarationForDefaultNullValueFixer - handle mixed pseudotype

[keradus]

closes #5483

[coveralls]

Coverage remained the same at 91.929% when pulling f12caa2 on keradus:2.18_fix5483 into ea98c01 on FriendsOfPHP:2.18.

[kubawerlos]

What about PHP < 8 where mixed is perfectly fine class name?

[keradus]

I believe we should not touch it, as if we would, we would make someone else code harder to migrate to PHP8.
Usage of future-reserved words, while possible on lower PHP engines, is discouraged.