Fix null Action.Title check in ChoiceFactory

[mdrichardson]

Fixes #1485

Per issue, if a Choice uses something like:

new Choice()
{
    Action = new CardAction()
    {
        Type = "messageBack",
        Value = "myValue",
        Title = "myTitle,
    }
}

instead of simply:

new Choice()
{
    Value = "myValue"
}

...it creates a Object reference not set to an instance of an object exception.

This is because if choice.Action.Title is null, this line:

var l = choice.Action != null && string.IsNullOrEmpty(choice.Action.Title) ? choice.Action.Title.Length : choice.Value.Length;

evaluates to:

var l = true && true = choice.Action.Title.Length

...so it checks for choice.Action.Title.Length when choice.Action.Title is null

[cleemullins]

Nice Fix.

However, please add a unit test that:

1. Fails before your change.
2. Passes after your change.

[mdrichardson]

@cleemullins

Before fix:

After: