specials: Clean up redirect special pages ($subpage can be null)

specials: Clean up redirect special pages ($subpage can be null)

$subpage being null is clearly documented in SpecialPage::run,
SpecialPage::execute, and most special page subclasses.

But all the redirect subclasses only copied part of the typehint,
making it look like it's always a string.

For SpecialMyLanguage, follows-up b1853bba0. Don't cast null to
empty string, and don't bother giving Title::newFromText an empty
string only to bail out with null again.

Also:
* Add visibility 'public' where missing.
* Add or correct relevant documentation comments.
* In SpecialMyRedirectPages, handle error first and avoid having
  final return inside a conditional; Remove redunant 'else'.

Change-Id: Ie3543f44011832b198bb3d3e32528b6a2868dee1