Page tree is not sorted by title when using Oracle DB

When giving pages titles different from their page names the pages are still sorted by their name (or the corresponding space name), at least in a single language wiki.

The reason for this is that in the SQL queries it is checked that doc.XWD_LANGUAGE = '' for the original (non-translated) page, while Oracle seems to store null instead in this field and needs a doc.XWD_LANGUAGE is null check instead.