fix: valid_date does not work in PHP 8.2

See php/php-src#9431 (comment)

Author: kenjis

system/Validation/FormatRules.php

@@ -344,6 +344,15 @@ public function valid_date(?string $str = null, ?string $format = null): bool
         $date   = DateTime::createFromFormat($format, $str);
         $errors = DateTime::getLastErrors();
 
-        return $date !== false && $errors !== false && $errors['warning_count'] === 0 && $errors['error_count'] === 0;
+        if ($date === false) {
+            return false;
+        }
+
+        // PHP 8.2 or later.
+        if ($errors === false) {
+            return true;
+        }
+
+        return $errors['warning_count'] === 0 && $errors['error_count'] === 0;
     }
 }