Complex object binding sends in null if any argument is evaluates to null (even w/ default)

Hi there, thanks for adding support for complex object binding -- makes the handler code SO much cleaner.

During my refactor to complex, I noticed that whenever a single argument is null w/o a default supplied (or default null), the entire complex type gets passed in as null.

I don't think passing in null would ever be desirable -- either the parser needs to throw or pass in the "default" version of the object (using `default(T)` for each argument that comes in as null maybe?)

We have plenty of use cases where we allow the user to not provide an input and we pull it from a saved setting.

Here's a working example of the behavior:

```csharp
class Program
{
static void Main(string[] args)
{
var root = new RootCommand();

root.AddCommand(TestCommand.Instance);

root.InvokeAsync(args).Wait();
}
}

public class TestRequest
{
public TestRequest(string test1)
{
this.Test1 = test1;
}
public string Test1 { get; }
}

public static class TestCommand
{
public static Command Instance = GetTestCommand();
private static Command GetTestCommand()
{
var test = new Command("test");

//broken w/ default null
//test.AddOption(new Option("--test1")
//{
// Argument = new Argument(() => null)
//});

////broken w/o default
//test.AddOption(new Option("--test1")
//{
// Argument = new Argument()
//});

//works w/ empty string
test.AddOption(new Option("--test1")
{
Argument = new Argument(() => "")
});

test.Handler = CommandHandler.Create((TestRequest request) =>
{
var msg = request == null ? "I'm broken!" : "I work!";

Console.WriteLine(msg);
});

return test;
}
}
```

Commencez par exécuter l’exemple de Program fourni avec TestRequest, TestCommand et CommandHandler.Create afin de reproduire le comportement de liaison de null. Examinez le chemin de liaison des objets complexes et ajoutez une couverture de régression pour les arguments manquants et ceux dont la valeur par défaut est null ; la finalisation nécessite un comportement convenu, soit rejeter l’entrée, soit construire la requête avec les valeurs d’argument par défaut.

Rédigé par le modèle d'indexation à partir du texte de l'issue.