[release-branch.go1.16] net: don't reject null mx records

Bypass hostname validity checking when a null mx record is returned as,
defined in RFC 7505.

Updates #46979
Updates #46999

Change-Id: Ibe683bd6b47333a8ff30909fb2680ec8e10696ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/332094


Trust: Roland Shoemaker <roland@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
(cherry picked from commit 03761ede)
Reviewed-on: https://go-review.googlesource.com/c/go/+/332371
Run-TryBot: Katie Hockman <katie@golang.org>