[8.x] Allow nullable columns for AsArrayObject/AsCollection casts

[tomirons]

When using either of these casts and allowing the column to be null it throws an undefined error whenever you access the attribute.

This fixes that issue.

[taylorotwell]

Is it possible they should just return null? What do the typical array and collection casts do?

[tomirons]

The typical ones will just return null, so I updated the casts to do the same.

[PhiloNL]

Hi @tomirons and @taylorotwell 🙌
I just updated one of my apps and this is a breaking change for me.
I have a user table with JSON settings column which is null by default:

Usage like this is no longer working:

auth()->user()->settings->get('mode', false);
auth()->user()->settings->put('hide_price', $state);

This now results in:

Call to a member function get() on null
Call to a member function put() on null

I think I can fix it by making the default column value {}. But again this might break applications who expected to always get a collection by default.

*Edit
JSON columns can't have default values 😅 so probably set it to an empty array during model creation.

Also mentioned here: #36730

I can make my own custom cast as suggested so I'm fine with this change but again, since the introduction of AsCollection, they returned a collection even if the value was null so more people might be affected.

[tomirons]

@PhiloNL this PR allows you to have null for these casts. It's up to you how the data is structured.