Why are there some downliks with no payload?

Hi,

I am testing multiple end devices on my desk at the same time. The payload is short and uplinked every 30 seconds. From time to time some of the devices receive downlink with zero length payload on port 0. In the console, the frame is shown as:
dev

then I checked the same data on the gateway and it looks like this:
gw

What are those data packets?

These are likely ADR commands. See TTN is trying to downlink {} to my node.

Thank you. Looks like those service frames.