Maybe a work around would be something like this:

SELECT *,

CASE WHEN val IS NULL THEN 0 ELSE 1 END,

NTILE(4) OVER (PARTITION BY CASE WHEN val IS NULL THEN 0 ELSE 1 END ORDER BY val) As Q

FROM #t;

Only use the rank when CASE WHEN val IS NULL THEN 0 ELSE 1 END = 1