[release-branch.go1.13] 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 #33984
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>
(cherry picked from commit
9d480edadc6144d9f9f5a896d729d1642e46083b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/192722