Fix collection parsing when the value is null

[tgeorgel]

Hey, thanks for your work on this :)

This PR fixes the issue when a collection field is created with null as default value :

Setting::name('lorem')->collection();

When using a Json Resource in a controller, which would convert the settings attribute to it's json representation, the following error is thrown :

json_decode(null, true, 512, JSON_THROW_ON_ERROR)

DEPRECATED  json_decode(): Passing null to parameter #1 ($json) of type string is deprecated in /var/www/marketing-smseval()'d code.

JsonException  Syntax error.

To address this issue, adding a default value to "{}" would parse the value as an empty collection :

json_decode($value ?? '{}', true, 512, JSON_THROW_ON_ERROR)
= []

It would be a nice idea to submit the package to packagist also.