[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 03761ede028d811dd7d7cf8a2690d4bfa2771d85)
Reviewed-on: https://go-review.googlesource.com/c/go/+/332371
Run-TryBot: Katie Hockman <katie@golang.org>
2 files changed
tree: 15a261ecfb293b9d98204ce49436d22d0ece6185
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
  18. SECURITY.md
  19. VERSION
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