[8.x] Fixes AsEncrypted traits not respecting nullable columns

[DarkGhostHunter]

What?

When using AsEncryptedCollection or AsEncryptedArrayObject, nulling the property of the model results in an error because the cast object returned isn't aware of null values being passed to or received from the database.

use Illuminate\Database\Eloquent\Casts\AsEncryptedArrayObject;
use Illuminate\Database\Eloquent\Casts\AsEncryptedCollection;
use Illuminate\Database\Eloquent\Model;

class Foo extends Model
{
    protected $casts = [
        'secret_array' => AsEncryptedArrayObject::class,
        'secret_collection' => AsEncryptedCollection::class,
    ]
}

Foo::make()->fill([
    'secret_array' => null,
    'secret_collection' => null,
])

// ErrorException : Undefined array key "secret_array"
// ErrorException : Undefined array key "secret_collection"

This fixes this behaviour by checking if the key in the attributes being passed to the cast object is not null. If it's not, proceeds to encrypt/decrypt the value, otherwise it keeps it as null.

Notes

There is an EXCESSIVE amount of redundant encryption being done when setting a value into a property. For example, saving the model calls the encrypted 4 times, when it only should be called once.

$foo = Foo::make();

$foo->secret_array = ['foo' => 'bar'];

$foo->save(); // <-- Here the value encryption is called 4 times.

You can check this behaviour on the tests themselves. The encryption must be set to be expected 12 times even if only it's saved twice in the database during a single test.