net: allow zero value destination address in WriteMsgUDPAddrPort
The existing address validity checks already cover both connected and
non-connected sockets. Pass a nil sockaddr just like WriteMsgUDP, when
the address is zero value.
TestWriteToUDP is extended to cover the netip APIs.
Fixes #74841
Change-Id: I2708e7747e224958198fe7abb3fcd8d59bc5a88a
Reviewed-on: https://go-review.googlesource.com/c/go/+/692437
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Sean Liao <sean@liao.dev> Reviewed-by: Damien Neil <dneil@google.com>