Correctly handle default values in batch update notifications.

We were previously calling `Optional.GetValueOrDefault<T>()` in order to convert an `Optional<object>` to an `Optional<T>` which meant that empty values were getting converted to default values, meaning that in certain cases change notifications weren't raised.

Add an `Optional<object>.Cast<T>()` methods (like we have in `BindingValue`) and use that in order to preserve empty values when ending a batch update operation.

src/Avalonia.Base/Data/Optional.cs

@@ -153,4 +153,18 @@ namespace Avalonia.Data
         /// </summary>
         public static Optional<T> Empty => default;
     }
+
+    public static class OptionalExtensions
+    {
+        /// <summary>
+        /// Casts the type of an <see cref="Optional{T}"/> using only the C# cast operator.
+        /// </summary>
+        /// <typeparam name="T">The target type.</typeparam>
+        /// <param name="value">The binding value.</param>
+        /// <returns>The cast value.</returns>
+        public static Optional<T> Cast<T>(this Optional<object> value)
+        {
+            return value.HasValue ? new Optional<T>((T)value.Value) : Optional<T>.Empty;
+        }
+    }
 }

src/Avalonia.Base/PropertyStore/BindingEntry.cs

@@ -127,8 +127,8 @@ namespace Avalonia.PropertyStore
             sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
                 owner,
                 (AvaloniaProperty<T>)property,
-                oldValue.GetValueOrDefault<T>(),
-                newValue.GetValueOrDefault<T>(),
+                oldValue.Cast<T>(),
+                newValue.Cast<T>(),
                 Priority));
         }
 

src/Avalonia.Base/PropertyStore/ConstantValueEntry.cs

@@ -65,8 +65,8 @@ namespace Avalonia.PropertyStore
             sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
                 owner,
                 (AvaloniaProperty<T>)property,
-                oldValue.GetValueOrDefault<T>(),
-                newValue.GetValueOrDefault<T>(),
+                oldValue.Cast<T>(),
+                newValue.Cast<T>(),
                 Priority));
         }
     }

src/Avalonia.Base/PropertyStore/LocalValueEntry.cs

@@ -36,8 +36,8 @@ namespace Avalonia.PropertyStore
             sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
                 owner,
                 (AvaloniaProperty<T>)property,
-                oldValue.GetValueOrDefault<T>(),
-                newValue.GetValueOrDefault<T>(),
+                oldValue.Cast<T>(),
+                newValue.Cast<T>(),
                 BindingPriority.LocalValue));
         }
     }

src/Avalonia.Base/PropertyStore/PriorityValue.cs

@@ -197,8 +197,8 @@ namespace Avalonia.PropertyStore
             sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
                 owner,
                 (AvaloniaProperty<T>)property,
-                oldValue.GetValueOrDefault<T>(),
-                newValue.GetValueOrDefault<T>(),
+                oldValue.Cast<T>(),
+                newValue.Cast<T>(),
                 Priority));
         }