Model.sum returns nil when no rows are selected. Intentional?

Is it intended that the AR sum method return nil rather than 0 (int) or
0.0 (float) when no rows match the conditions? This is the result I get
using MySQL.

I can't find any mention of this issue in either Trac or the mailing
list archives, so I'll make up a patch if there's confirmation this is
not the desired behaviour.