[11.x] Fix for breaking change in #53543 when ::withoutWrapping() is called after instance creation

[SanderMuller]

Fixes breaking change introduced in #53543

Comment finding the breaking change: #53543 (comment)

See failing test which is added to test the code below now is without a wrapper:

$resource = new PostResource(new Post([
    'id' => 5,
    'title' => 'Test Title',
]));
$resource::withoutWrapping();

This PR adds a check to determine if an explicit wrapper has been set on the instance, if not, use the logic from before #53543, this allows for static wrapper changes after the instance is created to have impact.

12.x consideration

It's worth considering making an intended breaking change in 12.x to no longer let static updates change the wrapper after instantiation, but this PR focuses on removing the breaking change I introduced with #53543

wrapper property consideration

Edit: this fix does introduce the side effect that manually setting $instance->wrapper = 'posts' will now have no effect, since that doesn't set the wrapperSet flag. Maybe the wrapper property should be protected/private with a getter (or async visibility in the future 👀)

[taylorotwell]

Reverted the original PR. We're going to revisit API resources in general for 12.x in February.