fix: correctly handle empty params

[kubuzetto]

Hi again;

It seems that I accidentally introduced a rather insidious bug with the PR #33.

If the --tags argument is not passed; the line below:

c := exec.Command("go", "list", tagsArg, a)

is executed as go list "" ./...; not go list ./.... It therefore tries to list the current folder. If there are no Go files directly in the current folder; this will cause the go list command to fail. Therefore we must only add tagsArg if it is specified.

The same bug already existed in a more hidden way, with how go list command can be replaced using GO_TEST_BINARY. If the new command is only one word; then this part would insert an empty string for the second word:

if len(gt) != 2 {
   gt = append(gt, "")
}

which is how I noticed this bug in the first place. Of course, I opened a PR that excludes empty strings in both cases: #36.