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

[macaws]

This PR attempts to address a problem when an entire column is null.

Problem

The current AsFluent implementation returns null when the database column is null, causing runtime errors when developers try to use Fluent methods:

// This crashes if preferences is null in database
$user->preferences->get('theme'); // Call to a member function get() on null

Rationale

By always returning a fluent, we can prevent errors when calling ->get() blindly. Since you can't set a default value for json columns in MySQL, it becomes particularly tricky to add this AsFluent cast to existing datasets.

This may be a "breaking" change if anyone is dependent on checking for null to determine if the column was set - but realistically, most developers want to work with an empty Fluent object rather than handle null checks.

[macaws]

Seems to be an issue with the test itself re: failing tests:

 docker.io/library/mysql:5.7
  /usr/bin/docker create --name 48c8cef7de12443b9d8df561ee116ba8_mysql57_72b477 --label a6ae00 --network github_network_122031dbbcd341cda0fcf19f9eaff7af --network-alias mysql -p 33306:3306 --health-cmd="mysqladmin ping" --health-interval=10s --health-timeout=5s --health-retries=3 -e "MYSQL_ALLOW_EMPTY_PASSWORD=yes" -e "MYSQL_DATABASE=forge" -e GITHUB_ACTIONS=true -e CI=true mysql:5.7
  51c0342d17ef850461b7982a72e48ede88a50abb6cb7202e150ffee1e650bd3d
  /usr/bin/docker start 51c0342d17ef850461b7982a72e48ede88a50abb6cb7202e150ffee1e650bd3d
  Error response from daemon: failed to set up container networking: driver failed programming external connectivity on endpoint 48c8cef7de12443b9d8df561ee116ba8_mysql57_72b477 (b18c4b55d5dc2693c09f5f0afbe71fc73fdd21cbc76fdca07925af5d527d0695): failed to bind host port for 0.0.0.0:33306:172.18.0.3:3306/tcp: address already in use
  Error: failed to start containers: 51c0342d17ef850461b7982a72e48ede88a50abb6cb7202e150ffee1e650bd3d

[taylorotwell]

This would technically be a breaking change as implemented. Note I think you could just do $user->preferences?->get('theme')