No, since there could be fields (like VARCHAR or BLOB) that don't accept 0 as value. In one query you could set every field to 0 (or something similar) if the field you check for in the row is null.
But you could use more than one query like
UPDATE tablename fieldname='0' WHERE fieldname=NULL
UPDATE tablename otherfield='0' WHERE otherfield=NULL


But don't rely on what I've said, I've never got deep into obscure SQL commands.