Followup r94349; Interwiki::getURL used `$title != null` to test if the $title arg...

Followup r94349; Interwiki::getURL used `$title != null` to test if the $title arg was passed and should be substituted. However `"" == null`, so as a result switching to using the argument broke [[mw:]] style interwiki links without an article title.
Update the Interwiki::getURL code to use isset(), and update the comment to tell pre-1.19 supporting extensions to do the entire urlencoding and $1 substitution on their own since Interwiki::getURL was essentially buggy and broken before now.