[9.x] Fix support of nullable type for AsArrayObject/AsCollection

[a-bashtannik]

Previous fix #36526 of these 2 classes not covered the behavior of php json_encode / json_decode functions when casting null.

// Bugly fragment
public function get($model, $key, $value, $attributes)
{
    // if $attributes[$key] === "null" it is assumed as "set" but ArrayObject can't accept null values.
    return isset($attributes[$key]) ? new ArrayObject(json_decode($attributes[$key], true)) : null;
}

If you have tried to set AsArrayObject attribute value to null (or had it by default), json_encode processed it as sting "null" which is correct from the point of JSON, and it was stored successfully. Nothing wrong here.

But next time you tried to access the attribute, the error ArrayObject::__construct(): Argument #1 ($array) must be of type array, null given was raised because it was already converted to a string and json_decode returns null taking "null" as an argument, which is not acceptable by ArrayObject. Collection class casts null to an empty array under the hood, there was no problem on practice, but I consider better to keep the same predictable behavior of get methods for both cast attribute classes.

This PR also fix potential issues with json_encode / json_decode casting boolean values.

[taylorotwell]

Why would it return an empty array object vs. null? It should just return null.

[a-bashtannik]

@taylorotwell it was a doubt, but I concluded it should be an empty ArrayObject because otherwise, it ruins the main purpose, and idea of AsArrayObject — casting the attribute to the object and allowing it to behave like an array.

You can't just call

$model->array_object['meta']['title'] = 'Developer';

Because array_object attribute is null.

You need to create something new

$model->array_object = ['meta' => ['title' => 'Developer']];

instead, but that's a bit against AsArrayObject idea, no?

Anyway, both variants are acceptable and close the bug.

[foremtehan]

#38570

[a-bashtannik]

@foremtehan you are right, docs are misleading, I followed them in the wrong direction designing the behavior of AsArrayObject/AsCollection.

Ok then, I will update this PR and docs. Should contain also one-two examples more.

$user = new User();
$user->options = ['foo' => ['key' => $value]];
$user->options['foo']['key'] = $value;

[AyubM]

Taylor, why should it return null instead of an empty ArrayObject? The whole point of casting a property as such is to allow subproperty modification.

As things are, $model->info['foo'] = 'bar' will fail unless the value had previously been set. Which, for new objects, isn't going to be the case.

Major inconvenience if it can return null -- devs either need to check and instantiate as an empty array before modifying any subproperties, or they will have to set the json column to default as {} in the migration as a hack. I spoke too soon, MySQL doesn't accept default values for JSON types. I posted as solution at #38570.