udp: make it possible to receive empty udp packets

A udp packet can have 0 content. In that case nread will be equal to 0,
but addr != NULL.

Add test case for empty data gram packets and fixed test that checked
for OOB when length == 0.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
lint: fix indentation
Merge remote-tracking branch 'upstream/master'