$test_mass['tk98op'] = array('p5ld2se'=>array(),
     '38hjp9'=>array('2we8t2'=>array(), '1142ln'=>array()),
     '669yo36'=>array('6318u7'=>array('22rv86'=>array(),
     '13we42'=>array()), 'jk66d3'=>array()));

function deep($mass, $b = 0)
    {
    foreach ($mass as $key=>$val)
        {
        if(is_array($val))
            {
            if(count($val) == 0)
                {
                /* не один из этих трех вариантов ничего не дописывает */
                //$val = array(1,4,217,432);
                //$mass = array_merge($val, array(1,4,217,432));
                //$mass[$key] = array(1,4,217,432);
                }
            deep($val, $b);
            }
        }
    return $mass;
    }

Array
(
    [tk98op] => Array
        (
            [p5ld2se] => Array
                (
                    [0] => 1
                    [1] => 4
                    [2] => 217
                    [3] => 432
                )
            [38hjp9] => Array
                (
                    [2we8t2] => Array
                        (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                    [1142ln] => Array
                        (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                )
            [669yo36] => Array
                (
                    [6318u7] => Array
                        (
                            [22rv86] => Array
                                (
                                    [0] => 1
                                    [1] => 4
                                    [2] => 217
                                    [3] => 432
                                )
                            [13we42] => Array
                                (
                                    [0] => 1
                                    [1] => 4
                                    [2] => 217
                                    [3] => 432
                                )
                        )
                    [jk66d3] => Array
                        (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                )
        )
)

$test_arr['tk98op'] = [
    'p5ld2se' => [],
    '38hjp9' => [
        '2we8t2' => [],
        '1142ln' =>[]
    ],
    '669yo36' => [
        '6318u7'=>[
            '22rv86'=>[],
            '13we42'=>[]
        ],
         'jk66d3'=>[]
    ]
];

deep($test_arr);

echo '<pre>', var_dump( $test_arr ), '</pre>';


function deep(&$arr) { // Передача аргумента по ссылке
    foreach ($arr as &$val) { // И в цикле тоже
        if (is_array($val)) {
            if (count($val) == 0) {
                $val = [1, 4, 217 , 432];
            }
            deep($val);
        }
    }
}

Array(
    [tk98op] => Array (
            [p5ld2se] => Array (
                    [0] => 1
                    [1] => 4
                    [2] => 217
                    [3] => 432
                )

            [38hjp9] => Array(
                    [2we8t2] => Array (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )

                    [1142ln] => Array (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                )

            [669yo36] => Array(
                    [6318u7] => Array(
                            [22rv86] => Array (
                                    [0] => 1
                                    [1] => 4
                                    [2] => 217
                                    [3] => 432
                                )

                            [13we42] => Array (
                                    [0] => 1
                                    [1] => 4
                                    [2] => 217
                                    [3] => 432
                                )
                        )

                    [jk66d3] => Array (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                )
        )
)