Fixed #103: Check for CommandParamenter value set instead of non-null…

… value

As it currently stands, the InvokeCommandAction.CommandParamenter can
never have a null value as it will make it fallback to the other
approaches, which can raise an InvalidCastException.

A null value should be allowed for the CommandParameter, so instead of
doing a null check, I've replaced it with a property unset check
instead.

src/BehaviorsSDKManaged/Microsoft.Xaml.Interactions/Core/InvokeCommandAction.cs

@@ -158,7 +158,7 @@ public object Execute(object sender, object parameter)
             }
 
             object resolvedParameter;
-            if (this.CommandParameter != null)
+            if (this.ReadLocalValue(InvokeCommandAction.CommandParameterProperty) != DependencyProperty.UnsetValue)
             {
                 resolvedParameter = this.CommandParameter;
             }