Fix scope with state zero

[mvdnbrk]

I had two states, let's say Draft and Published with values of 0 and 1:

class Draft extends PostState
{
    public static $name = 0;
}

// and

class Published extends PostState
{
    public static $name = 1;
}

Now when you scope the model like so:

Post::whereState('state', Draft::class)

This will return the published ones instead of the expected posts with state Draft.
This is because of the $name property in the Draft state is set to 0.

In this scenario the state class is not resolved properly in the resolveStateClass method.

This PR aims to solve this issue.
Added a test as well.

[brendt]

Thanks!

[brendt]

Released as 1.6.3: https://github.com/spatie/laravel-model-states/releases/tag/1.6.3