net: return nil UDPAddr from ReadFromUDP

In cases where the socket operation has no underlying address,
golang.org/cl/291509 unintentionally changed ReadFromUDP from return a
nil *UDPAddr to a non-nil (but zero value) *UDPAddr.

This may break callers that assume "no address" is always addr == nil,
so change it back to remain nil.

Fixes #46238

Change-Id: I8531e8fa16b853ed7560088eabda0b9e3e53f5be
Reviewed-on: https://go-review.googlesource.com/c/go/+/320909


Trust: Michael Pratt <mpratt@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>