[8.x] Loose comparison causes the value not to be saved

[vsg24]

The PR back-ports #41337 to 8.x branch.

The description below is copied from the original PR.

Given an attribute with the cast object or collection, the $model->isDirty() method returns the wrong value due to the use of loose comparison.

Example from issue #37961:

use Illuminate\Database\Eloquent\Model;

class Test extends Model
{
    protected $casts = [
        'collection' => 'collection',
    ];
    protected $fillable = [
        'collection',
    ];
}

$model = new Test(['collection' => ['key' => true]]);
$model->syncOriginal();
$model->fill(['collection' => ['key' => 'value']]);

$model->isDirty(); // false but should be true

// or
$model = new Test(['collection' => ['key' => null]]);
$model->syncOriginal();
$model->fill(['collection' => ['key' => false]]);

$model->isDirty(); // false but should be true

[MasterRO94]

This PR breaks BC. If original $value is null then PHP Deprecated: json_decode(): Passing null to parameter #1 ($json) of type string is deprecated is thrown