fix(serialization): correct validation parameter

- Fix bug in mapUnmarshalValidate where checkValidateTag parameter
  was incorrectly negated when passed to Convert()
- Remove obsolete validateWithValidator helper function

internal/serialization/validation.go

@@ -29,17 +29,19 @@ type CustomValidator interface {
 var validatorType = reflect.TypeFor[CustomValidator]()
 
 func ValidateWithCustomValidator(v reflect.Value) gperr.Error {
+	vt := v.Type()
 	if v.Kind() == reflect.Pointer {
-		if v.IsNil() {
-			// return nil
-			return validateWithValidator(reflect.New(v.Type().Elem()))
-		}
-		if v.Type().Implements(validatorType) {
+		elemType := vt.Elem()
+		if vt.Implements(validatorType) {
+			if v.IsNil() {
+				return reflect.New(elemType).Interface().(CustomValidator).Validate()
+			}
 			return v.Interface().(CustomValidator).Validate()
 		}
-		return validateWithValidator(v.Elem())
+		if elemType.Implements(validatorType) {
+			return v.Elem().Interface().(CustomValidator).Validate()
+		}
 	} else {
-		vt := v.Type()
 		if vt.PkgPath() != "" { // not a builtin type
 			// prioritize pointer method
 			if v.CanAddr() {
@@ -56,10 +58,3 @@ func ValidateWithCustomValidator(v reflect.Value) gperr.Error {
 	}
 	return nil
 }
-
-func validateWithValidator(v reflect.Value) gperr.Error {
-	if v.Type().Implements(validatorType) {
-		return v.Interface().(CustomValidator).Validate()
-	}
-	return nil
-}