supporting null or string.empty to trigger property notifications.

src/Markup/Perspex.Markup/Data/Plugins/InpcPropertyAccessorPlugin.cs

@@ -105,7 +105,7 @@ namespace Perspex.Markup.Data.Plugins
 
             private void PropertyChanged(object sender, PropertyChangedEventArgs e)
             {
-                if (e.PropertyName == _property.Name)
+                if (e.PropertyName == _property.Name || string.IsNullOrEmpty(e.PropertyName))
                 {
                     _changed(Value);
                 }

tests/Perspex.Markup.UnitTests/Data/ExpressionObserverTests_Property.cs

@@ -101,7 +101,10 @@ namespace Perspex.Markup.UnitTests.Data
             var sub = target.Subscribe(x => result.Add(x));
             data.Foo = "bar";
 
-            Assert.Equal(new[] { "foo", "bar" }, result);
+            data.RaisePropertyChanged(string.Empty);
+            data.RaisePropertyChanged(null);
+
+            Assert.Equal(new[] { "foo", "bar", "bar", "bar" }, result);
 
             sub.Dispose();
 
@@ -333,6 +336,11 @@ namespace Perspex.Markup.UnitTests.Data
                     RaisePropertyChanged(nameof(Next));
                 }
             }
+
+            public void RaisePropertyChanged(string propertyName)
+            {
+                base.RaisePropertyChanged(propertyName);
+            }
         }
 
         private class Class2 : NotifyingBase, INext