Fix cast with null

[bastien-phi]

Currently, a DTO like

class ComplexDtoWithNullableProperty extends DataTransferObject
{
    public string $name;

    public ?BasicDto $other;
}

cannot be created with

new ComplexDtoWithNullableProperty(name: 'a', other: null)

because we are having a

TypeError: Only arrays and Traversables can be unpacked

vendor/spatie/data-transfer-object/src/Casters/DataTransferObjectCaster.php:17
vendor/spatie/data-transfer-object/src/Reflection/DataTransferObjectProperty.php:41

This PR fixes the issue applying the cast only when given value is not null.

[brendt]

Aah, yes, thanks for sending a PR, gonna tag immediately!

[brendt]

Done! https://github.com/spatie/data-transfer-object/releases/tag/3.0.1

[bastien-phi]

Wait a minute, I'm just sending another PR

[bastien-phi]

You're too quick for me ! 😂

[brendt]

I don't mind many tags ;)