Not necessarily...with no payload, the endurance would stretch out quite a bit. Normal payload/range curves don't usually go down as low as "fuel fuel / no pax / no cargo" because it's not a commercially viable configuration.
Quoting CPH-R (Reply 1): Looking at the speed graph, could it be stall-testing of sorts?
There are no associated altitude drops. Plus, for stalls, you'd want to be in reserved airspace and that flight track is huge...someone more familiar with the area can advise, but I strongly suspect you can't get a reserved block that large that close to DAL.