syscall: permit nil destination address in sendmsgN{Inet4,Inet6}

syscall: permit nil destination address in sendmsgN{Inet4,Inet6}

For #74841

Change-Id: Ic3ef9c61a89bc8bc0594724a6d94eed91fb63eae
Reviewed-on: https://go-review.googlesource.com/c/go/+/692435
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>