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 issue 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://codereview.appspot.com/12916043