cmd/go: fix bad error message in coverage for package without non-test files

Was checking for nil map; must check for empty map instead.

Fixes #6065

Before:

go test -cover
# testmain
/var/folders/00/013l0000h01000cxqpysvccm0004fc/T/go-build233480051/_/Users/r/issue/_test/_testmain.go:11: imported and not used: "_/Users/r/issue"
FAIL	_/Users/r/issue [build failed]

Now:

go test -cover
testing: warning: no tests to run
PASS
coverage: 0.0% of statements
ok  	_/Users/r/issue	0.021s

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12916043

src/cmd/go/test.go

@@ -1010,7 +1010,7 @@ type coverInfo struct {
 func writeTestmain(out string, pmain, p *Package) error {
 	var cover []coverInfo
 	for _, cp := range pmain.imports {
-		if cp.coverVars != nil {
+		if len(cp.coverVars) > 0 {
 			cover = append(cover, coverInfo{cp, cp.coverVars})
 		}
 	}