Allow .sort() descending to specify null at bottom instead of top

[bill-zhanxg]

Is your feature request related to a problem? Please describe.

Allow .sort() descending to specify null at bottom instead of top, this is useful when you're trying to sort conversations with latest_message linked field and it might be null

Describe the solution you'd like

db.sort(column, direction, nullAtBottom)

Describe alternatives you've considered

There is no alternative for this