How to display everything using content filtering?

Question

AJAX

I have a content filter on the page, select the area from the drop-down list and get posts with the specified area, but if we select the default value (select the area), then in theory it should give all the posts, but does not give out, how to fix it? You can catch this value with all, but I don’t know how to display all posts.

<form action="https://....ru/wp-admin/admin-ajax.php" method="POST" id="filter">
              <select class="doc__select" name="vdselect" >
                                  <option value="all">Выберите район</option>
                                  <option value="Адмиралтейский">Адмиралтейский</option>
                                  <option value="Василеостровский">Василеостровский</option>

                              </select>
              <input type="hidden" name="action" value="myfilter">
          </form>

jQuery(document).ready(function($) {
  $('select.doc__select').change(function(){	
    // Узнаем выбранный район
    var city =  $(this).find("option:selected").val(); 
    
    if (city == 'all') {

    }
       
      var filter = $('#filter');
      //filter.submit();
    $.ajax({
      url:filter.attr('action'),
      data:filter.serialize(), // form data
      type:filter.attr('method'), // POST
      beforeSend:function(xhr){
        
      },
      success:function(data){
        
        $('#response').html(data); // insert data
      }
    });
    return false;
     
  });
});