Fixed #1735 Return empty []string

Need return empty []string  if config value is empty.

split `“”` ==> []string{}, Not []string{“”}

config/yaml/yaml_test.go

@@ -79,4 +79,8 @@ func TestYaml(t *testing.T) {
 	if yamlconf.String("name") != "astaxie" {
 		t.Fatal("get name error")
 	}
+
+	if len(yamlconf.Strings("emptystrings")) != 0 {
+		t.Fatal("get emtpy strings error")
+	}
 }