Fix: BindTo and OneWayBind not working for null values

src/ReactiveUI.Tests/Platforms/windows-xaml/PropertyBindingTest.cs

@@ -235,7 +235,7 @@ public void ViewModelIndexerPropertyToView()
         }
 
         [Fact]
-        public void BindToShouldntInitiallySetToNull()
+        public void OneWayBindShouldntInitiallySetToNull()
         {
             var vm = new PropertyBindViewModel();
             var view = new PropertyBindView { ViewModel = null };
@@ -354,6 +354,77 @@ public void OneWayBindConverter()
             Assert.False(view.SomeTextBox.IsEnabled);
         }
 
+        [Fact]
+        public void OneWayBindWithNullStartingValueToNonNullValue()
+        {
+            var vm = new PropertyBindViewModel();
+            var view = new PropertyBindView { ViewModel = vm };
+
+            view.OneWayBind(vm, x => x.Property1, x => x.SomeTextBox.Text);
+
+            vm.Property1 = "Baz";
+
+            Assert.Equal("Baz", view.SomeTextBox.Text);
+        }
+
+        [Fact]
+        public void OneWayBindWithNonNullStartingValueToNullValue()
+        {
+            var vm = new PropertyBindViewModel();
+            var view = new PropertyBindView { ViewModel = vm };
+
+            vm.Property1 = "Baz";
+
+            view.OneWayBind(vm, x => x.Property1, x => x.SomeTextBox.Text);
+
+            vm.Property1 = null;
+
+            Assert.True(string.IsNullOrEmpty(view.SomeTextBox.Text));
+        }
+
+        [Fact]
+        public void OneWayBindWithSelectorAndNonNullStartingValueToNullValue()
+        {
+            var vm = new PropertyBindViewModel();
+            var view = new PropertyBindView { ViewModel = vm };
+
+            view.OneWayBind(vm, x => x.Model, x => x.SomeTextBox.Text, x => x?.AnotherThing);
+
+            vm.Model = null;
+
+            Assert.True(string.IsNullOrEmpty(view.SomeTextBox.Text));
+        }
+
+        [Fact]
+        public void BindToWithNullStartingValueToNonNullValue()
+        {
+            var vm = new PropertyBindViewModel();
+            var view = new PropertyBindView { ViewModel = vm };
+
+            view.WhenAnyValue(x => x.ViewModel!.Property1)
+                .BindTo(view, x => x.SomeTextBox.Text);
+
+            vm.Property1 = "Baz";
+
+            Assert.Equal("Baz", view.SomeTextBox.Text);
+        }
+
+        [Fact]
+        public void BindToWithNonNullStartingValueToNullValue()
+        {
+            var vm = new PropertyBindViewModel();
+            var view = new PropertyBindView { ViewModel = vm };
+
+            vm.Property1 = "Baz";
+
+            view.WhenAnyValue(x => x.ViewModel!.Property1)
+                .BindTo(view, x => x.SomeTextBox.Text);
+
+            vm.Property1 = null;
+
+            Assert.True(string.IsNullOrEmpty(view.SomeTextBox.Text));
+        }
+
         [Fact]
         public void BindExpectsConverterFuncsToNotBeNull()
         {

src/ReactiveUI/Bindings/Property/PropertyBinderImplementation.cs

@@ -319,7 +319,7 @@ public IDisposable BindTo<TValue, TTarget, TTValue>(
 
                 if (converter == null)
                 {
-                    defaultSetter?.Invoke(paramTarget, paramValue ?? throw new ArgumentNullException(nameof(paramValue)), paramParams);
+                    defaultSetter?.Invoke(paramTarget, paramValue, paramParams);
                     return defaultGetter(paramTarget, paramParams);
                 }