spec: describe an edge case of slice-to-array conversions

spec: describe an edge case of slice-to-array conversions

Converting from nil slice to zero element array is ok, so explicitly
describe the behavior in the spec.

For #46505

Change-Id: I68f432deb6c21a7549bf7e870185fc62504b37f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/430835
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>