[Observium] device_rebooted metric is now NULL instead of 1

We have an alert checker using the device_rebooted metric to monitor for reboots. The metric used to read as 1 when a reboot was detected as is documented https://docs.observium.org/entities/#device. After updating to the latest stable revision (r13020) from r12520 the metric now reads as NULL immediately after a reboot. Is this change in behavior intended?

Corwin