Filter search result by either a taxonomy1, or a taxonymy2, or both taxonomy1 and taxonomy2 - how to modify this code?

Hi guys,

I have a problem which is driving me crazy and I hope someone will point me in a good direction.

I have a custom post type called Restaurants with two taxonomies Cities and Cuisine Type. I can select with checkboxes city/cities and cuisine type/types and it shows me all restaurants in selected options.

What doesn't work is selection only by a cuisine type or only by a city, so:

1. If I check for example Chicago, italian food and french food - it shows all the restaurants with selected options

2. But if I check only a city, or only a cuisine type - I get no results.

I think it's messed with "if" statement but I'm not a php master and can't figure out how to clean it... The code is:

<?php 

$search_city = true;
$cuisine = array_keys($_GET['cuisine-type']);

    if(isset($_GET['city'])){
        $city = array_keys($_GET['city']);
    }else{
        $city = array();
    }


    $loop = new WP_Query(array(
        'post_type' => 'restaurants',
        'tax_query' => array(
            'relation' => 'AND',
                array(
                'taxonomy' => 'city',
                'field'    => 'slug',
                'terms' => $city,
                ),  
                array(
                'taxonomy' => 'cuisine-type',
                'field' => 'slug',
                'terms' => $cuisine,
                ),
        ),
    ));

    get_template_part('archive', 'restaurants');

?>

If I remove 'relation' => 'AND' then I can search only by a city or by a cuisine type, when both city and cuisine-type are selected, the results will not show up.