Code refactoring (not a fan of StyleCop here)

Author: Sergio0694

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableValidator.cs

@@ -253,11 +253,6 @@ private void ValidateProperty(object? value, string? propertyName)
         {
             if (propertyName is null)
             {
-                static void ThrowArgumentNullExceptionForNullPropertyName()
-                {
-                    throw new ArgumentNullException(nameof(propertyName), "The input property name cannot be null when validating a property");
-                }
-
                 ThrowArgumentNullExceptionForNullPropertyName();
             }
 
@@ -296,5 +291,15 @@ static void ThrowArgumentNullExceptionForNullPropertyName()
                 ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
             }
         }
+
+#pragma warning disable SA1204
+        /// <summary>
+        /// Throws an <see cref="ArgumentNullException"/> when a property name given as input is <see langword="null"/>.
+        /// </summary>
+        private static void ThrowArgumentNullExceptionForNullPropertyName()
+        {
+            throw new ArgumentNullException("propertyName", "The input property name cannot be null when validating a property");
+        }
+#pragma warning restore SA1204
     }
 }