[11.x] Add whenExistsLoaded method to conditionally include relationship existence attribute

[CodeWithKyrian]

Introduction

This PR introduces a new method, whenExistsLoaded, to the Illuminate\Http\Resources\ConditionallyLoadsAttributes trait. This method allows developers to conditionally include a relation existence attribute in their API resources if it was eager loaded.

Background

Laravel's JsonResource currently provides several methods to conditionally include data in API responses based on whether certain relationships or attributes have been loaded. These methods include:

• whenLoaded: to include a relationship if it was eager loaded.
• whenCounted: to include a relationship count if it was eager loaded.
• whenAggregated: to include an aggregated value if it was eager loaded.

However, there is currently no method for conditionally including a relation existence attribute that has been eager loaded. This feature addresses that gap, providing a consistent API for conditional inclusion of various types of data.

Usage

Here is an example of how this new method can be used:

public function toArray(Request $request)
{
    return [
        'id' => $this->id,
        'name' => $this->name,
        'relation_exists' => $this->whenExistsLoaded('relation'),
    ];
}

[taylorotwell]

Please remove all of the white space related changes.