Motivation:
Datagrams are allowed to be empty when sent over UNIX or INET sockets.
In such cases, we cannot use "received zero bytes" as a signal that no packets were received.

Modification:
When we receive a zero-sized datagram, tell the EpollRecvByteAllocatorHandle that we really received one byte, because the allocation handles use zero as an end-of-data signal.
Such signals will put the event loop to rest, which is wrong because we don't know at this point if there are more packets to process.
Use the absence of a sender-point as signal that no packets were read from recvmsg(2), and then continue to use -1 as the end-of-data signal to allocation handle.
Add a DatagramUnicastTest to verify that we can receive multiple empty datagrams in a row without the eventloop mistakenly going to sleep.

Result:
Robust handling of zero-sized datagrams in the epoll transport.

…does not support domain sockets

…mPacket does not support domain sockets"

This reverts commit a443211.

… the JDK DatagramPacket class

As of JDK 20, it still does not support UNIX addresses, but some day it might.

…er, and not a mix

… the JDK