Return nil not empty []string{}

Return nil if config value does not exist or is empty

config/xml/xml_test.go

@@ -82,7 +82,7 @@ func TestXML(t *testing.T) {
 	if xmlconf.String("name") != "astaxie" {
 		t.Fatal("get name error")
 	}
-	if len(xmlconf.Strings("emptystrings")) != 0 {
+	if xmlconf.Strings("emptystrings") != nil {
 		t.Fatal("get emtpy strings error")
 	}
 }