crypto/tls: make config.Clone return nil if the source is nil
Fixes #40565
Change-Id: I13a67be193f8cd68df02b8729529e627a73d364b
GitHub-Last-Rev:
b03d2c04fd88db909b40dfd7bd08fe13d8994ab9
GitHub-Pull-Request: golang/go#40566
Reviewed-on: https://go-review.googlesource.com/c/go/+/246637
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>