bug#62990: 30.0.50; UDP server closes connection upon receiving an empty packet

[Eli Zaretskii]

> Cc: 62990@debbugs.gnu.org
> From: Robert Pluim <rpluim@gmail.com>
> Date: Wed, 26 Apr 2023 11:14:42 +0200
> 
> So allowing emacs to send 0-length UDP packets turned out to be not
> too hard. Patch for both below. Eli, is this OK for master? It does
> change 2 behaviours:
> 
> 1. Receiving a 0-length UDP packet will no longer cause an error
> 2. Calling (process-send-string proc "") now actually sends out a
> packet for a UDP process (an empty one).

Do we want some variable to get back the old behavior, in case some
application out there depends on that?

Otherwise, fine by me, thanks.