So Oracle has NULLS FIRST, which I can use to have null values sorted at the top followed by my column value in descending order:
ORDER BY date_sent NULLS FI
You can't control this, to my knowledge. And it looks like you have the correct approach with ISNULL.
ISNULL
With strings, I've used ISNULL(field, '') for the same purpose.
ISNULL(field, '')