common: don't scrub ""

If the access_key or secret_key were loaded from
somewhere other than the packer file then
ScrubConfig can get called to scrub "" and "".

This results in very long output:

<Filtered><<Filtered>F<Filtered>i...

Don't do that.

common/config.go

@@ -18,6 +18,9 @@ import (
 func ScrubConfig(target interface{}, values ...string) string {
 	conf := fmt.Sprintf("Config: %+v", target)
 	for _, value := range values {
+		if value == "" {
+			continue
+		}
 		conf = strings.Replace(conf, value, "<Filtered>", -1)
 	}
 	return conf