OutputPage: Only set <link hreflang=x-default> on wiki/pages with variants
Having the default variant advertised seems useful, however for
wikis and/or pages that have no variants or translated versions,
outputting this header doesn't seem useful.
justify sending it unconditionally.
To try out, setting $wgLanguageCode = 'zh'; will result in all
pages having hreflang=x-default and all variants. And setting to
'en' or 'de' will result in none of those link tags.