[13.x] Always cast something as fluent if column is null

[macaws]

This is a resubmission of PR #56540, previously closed as a breaking change for Laravel 12.x. Proposing for Laravel 13.

Problem:

The AsFluent cast returns null when a database column is null, requiring null checks:

$user->preferences->get('theme'); // Crashes if preferences is null
$user->preferences?->get('theme'); // Current workaround - seems out of character for "Fluent"

Solution:

Always return a Fluent instance, even when the column is null:

$user->preferences->get('theme'); // Works safely, returns null if empty

This is especially useful for JSON columns in MySQL where default values cannot be set at the database level.

Breaking Change:

Code that explicitly checks for null to determine if a column was set will break. Acceptable for a major version release.

[taylorotwell]

I don't think the breaking change is worth it to be honest. You can write a custom cast for this behavior if you want!