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
Use Case/When statement, for example:
ORDER BY (case WHEN ColINT IS NULL THEN {maxIntValue} ELSE ColINT END) DESC
ORDER BY (case WHEN ColVChar IS NULL THEN {maxVCharValue} ELSE ColVChar END) DESC
ORDER BY (case WHEN ColDateT IS NULL THEN {maxDateTValue} ELSE ColDateT END) DESC
...and so on.
or even better as you don't care what is your column type and the max value.
ORDER BY (case WHEN ColAnyType IS NULL THEN 1 ELSE 0 END) DESC, ColAnyType DESC