set test args to be empty, not nil

if nil, the args will grab from os.Args which isn't what we want here
diff --git a/docopt_test.go b/docopt_test.go
index 4cdc6ac..209c524 100644
--- a/docopt_test.go
+++ b/docopt_test.go
@@ -1204,7 +1204,7 @@
 	if v, _, err := parse("usage: prog [-v]", []string{"-v"}, true, "", false); reflect.DeepEqual(v, map[string]interface{}{"-v": true}) != true {
 		t.Error(err)
 	}
-	if v, _, err := parse("usage: prog [-vv]", nil, true, "", false); reflect.DeepEqual(v, map[string]interface{}{"-v": 0}) != true {
+	if v, _, err := parse("usage: prog [-vv]", []string{}, true, "", false); reflect.DeepEqual(v, map[string]interface{}{"-v": 0}) != true {
 		t.Error(err)
 	}
 	if v, _, err := parse("usage: prog [-vv]", []string{"-v"}, true, "", false); reflect.DeepEqual(v, map[string]interface{}{"-v": 1}) != true {