crypto/x509: allow nil Certificates to be compared in Equal

The current implementation panics on nil certificates,
so introduce a nil check and early return true if both
are nil, false if only one is.

Fixes #28743

Change-Id: I71b0dee3e505d3ad562a4470ccc22c3a2579bc52
Reviewed-on: https://go-review.googlesource.com/c/go/+/167118


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>