Arr::add()
The Arr::add method adds a given key / value pair to an array if the given key doesn't already exist in the array or is set to null:

use Illuminate\Support\Arr;

$array = Arr::add(['name' => 'Desk'], 'price', 100);

// ['name' => 'Desk', 'price' => 100]

$array = Arr::add(['name' => 'Desk', 'price' => null], 'price', 100);

// ['name' => 'Desk', 'price' => 100]