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/+/192340Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/cover/func.go

@@ -191,6 +191,10 @@ func findPkgs(profiles []*Profile) (map[string]*Pkg, error) {
 		}
 	}
 
+	if len(list) == 0 {
+		return pkgs, nil
+	}
+
 	// Note: usually run as "go tool cover" in which case $GOROOT is set,
 	// in which case runtime.GOROOT() does exactly what we want.
 	goTool := filepath.Join(runtime.GOROOT(), "bin/go")

src/cmd/go/testdata/script/cover_mod_empty.txt

+go tool cover -func=cover.out
+stdout total.*statements.*0.0%
+
+go mod init golang.org/issue/33855
+
+go tool cover -func=cover.out
+stdout total.*statements.*0.0%
+
+-- cover.out --