go/types, types2: first argument to copy must never be nil

Same fix as CL 718860, but for copy builtin.

Fixes #79687

Change-Id: Ia4c5fadf01a429a100c676e3854c569f06ea0904
Reviewed-on: https://go-review.googlesource.com/c/go/+/783640
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>