Null values cannot be filtered

•     this.api().columns([2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33]).every( function () {             
                  var column = this;
                  var select = $('<select style="border:1px solid #ced4da;height:30px"><option value="">全部</option></select>')
                      .appendTo( $(column.footer()).empty() )
                      .on( 'change', function () {
                          var val = $.fn.dataTable.util.escapeRegex(
                              $(this).val()
                          );
  
                          column
                              .search( val ? '^'+val+'$' : '', true, false )
                              .draw();
                      } );
  
                  column.data().unique().sort().each( function ( d, j ) {
                   if(column.search() === '^'+d+'$'){
          select.append( '<option value="'+d+'" selected="selected">'+d+'</option>' )
      } else {
          select.append( '<option value="'+d+'">'+d+'</option>' )
      }
                  } );
              } );
          },**: 
  

There are null values and null values in my data. Other values can be filtered by drop-down, but null values and null values cannot be filtered. Is there a problem with my local code? Please help correct it.