AppEUI == 0 issues with TTN V2/V3 migration

See arduino-lmic #759.

I saw an issue with using AppEUI==0 for uplinks when migrating a V2 device to a US V3 app, using a V2 gateway.

I set the AppEUI of the device to zero to move it to V3 (also adding it on the V3 console).

Device claimed to be joining (in the serial logs). But the TTN V2 console for the gateway didn’t show any uplinks. After a lot of headscratching, changed the AppEUI to 1 in the device… and things worked immediately.

I suspected an LMIC bug, because I remembered others reporting this problem. But I’ve tested with an RWC5020B network emulator, and all is well. I also checked with @tanupoo’s lorawan-parser, and all was well.

So now it appears either to be a gateway problem or a TTN V2 problem. I will continue investigations and post results here.