Sorting With 'desc Nulls Last'

PostgreSQL knows the sorting option

SORT BY row DESC NULLS LAST

That means, that rows with NULL get sorted behind 1 or 0 for example. The default instead is, that all NULLs get sorted in front of 1 or 0 which is annoying in my case.

Now, CSqlDataProvider doesn't seem to know that sorting and I am wondering how can I modify the code of yii to add the NULLS LAST thing behind every SORT BY row DESC (maybe even permanently).
Can anyone help me with that?