fix: treat null/undefined contact strings as empty

Co-authored-by: Cursor <cursoragent@cursor.com>

src/Oms.Application/Shared/CustomerContactHelper.cs

@@ -30,6 +30,11 @@ public static class CustomerContactHelper
     public static string? NormalizeOptionalContact(string? value)
     {
         var trimmed = value?.Trim();
-        return string.IsNullOrWhiteSpace(trimmed) ? null : trimmed;
+        if (string.IsNullOrWhiteSpace(trimmed)) return null;
+        // 前端/第三方有时会把缺失值以字符串形式传过来（如 "null" / "undefined"）
+        // 这类值应按“未填写”处理，否则会误触发“已被占用”校验。
+        if (trimmed.Equals("null", StringComparison.OrdinalIgnoreCase)) return null;
+        if (trimmed.Equals("undefined", StringComparison.OrdinalIgnoreCase)) return null;
+        return trimmed;
     }
 }