忽略数组codeigniter中的空值

Question

我想使用php从多个选择中填充我的下拉菜单，但是当我的查询的某些值返回null时，下拉菜单无法获取整体值，但我遇到了问题。这是我的数组json编码的示例。

[{"A"},{"B"}][][][][{"D"},{"E"},{"F"},{"G"},{"H"}][{"I"},{"J"}]

这是我在控制器上的查询代码

for ($i=0; $i<count($customerField); $i++) { 
        for ($j=0; $j<count($projectField); $j++) { 
            for ($k=0; $k<count($user_roleField); $k++) { 
                $customer = $customerField[$i];
                $project  = $projectField[$j];
                $user_role = $user_roleField[$k];

                $query = $this->db->query("SELECT user_id
                FROM  `ixt_user_project_list` 
                LEFT JOIN ixt_user_type ON ixt_user_project_list.user_type = ixt_user_type.user_type
                WHERE ixt_user_project_list.user_cust_id ='".$customer."'
                AND ixt_user_project_list.user_project_id ='".$project."'
                AND ixt_user_type.user_owner ='".$user_role."'")->result_array();

                echo json_encode($query);
            }
        }
    }

这是我的js视图

$('select#roleField').on('change', function () {
            var customer = $('select#customerField').val();
            var project = $('select#projectField').val();
            var role = $('select#roleField').val();
            var data = {
              customer: customer,
              project: project,
              role: role,
              "<?=$this->security->get_csrf_token_name()?>" : "<?=$this->security->get_csrf_hash()?>"
            };
            $.post('<?php echo base_url(); ?>Dashboard/get_userid', data, function(json) {
                userid_data = '<option value="All" selected>All</option>';
                $.each(json, function(index, obj){
                      userid_data += '<option value="'+obj.user_id+'">'+obj.user_id+'</option>';    
                });
                $userid.html(userid_data);
            }, 'JSON');
        });

有人可以帮助我解决我的问题吗？谢谢。