I can send and receive packets with payload size of zero on master (gnrc_networking+udp).
However, with this PR I can still send packets with 0 payload, but I am not receiving these packets. Reception of packets with some payload still works. I think this is a step backwards.?

it should be the other way around, ideed... Will have another look

This one is the only one remaining in #11163. I try to test this tomorrow. Or did you figure out what went wrong @SemjonKerner?

Please try it. I was not in office last week and will not come in before thursday, so I could not verify my findings. In the worst case I messed up with github and in the best case the master is already fit for #11163.

I can confirm @SemjonKerner's findings, that in master sending 0 bytes is possible, but with this PR it is not (I even checked the debug output in the driver; the packet does not arrive there). As to why this is: This PR is based on a version of master before #11161 was merged, which only allowed GNRC to handle payload length 0 packets in the first place (as pointed out in #11163). Considering that #11163 is not an issue for nrfmin in master, I rather close this PR as it adds unnecessary code.