cmd/cover: skip go list when profile is empty

Only call "go list" when explicitly listing packages. An empty coverage
profile references no packages, and would otherwise lead to "go list"
implicitly looking at the package in "." (which might not exist).

Fixes #33855

Change-Id: I02d4e374405d86f03d105fe14648aa03b4d2284c
Reviewed-on: https://go-review.googlesource.com/c/go/+/192340


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>