#62023 [13.x] Fix mergeWhen() in JSON:API resource attributes

Using mergeWhen() or merge() in a JSON:API resource's toAttributes() throws on every request:

class InvoiceResource extends JsonApiResource
{
    public function toAttributes(Request $request): array
    {
        return [
            'number' => $this->number,
            'total' => $this->total,
            $this->mergeWhen($request->user()->isAccountant(), [
                'internal_notes' => $this->internal_notes,
            ]),
        ];
    }
}

Error: Object of class Illuminate\Http\Resources\MergeValue could not be converted to string