Mysql将0更改为NULL

I have text field in my database table. It will contain, some content of my website. This content has also some opening hours. And during the updating the field mysql changes 9:00 to 9NULL. How can I prevent this ?

this is my update query:

 $data = $db->query("UPDATE `pages` SET `pl`='$pl' WHERE `id`='$id'");

EDIT:

TABLE STRUCTURE:

  `id` int(11) NOT NULL,
  `name` text COLLATE utf8mb4_bin NOT NULL,
  `pl` text COLLATE utf8mb4_bin,
  `en` text COLLATE utf8mb4_bin NOT NULL,
  `de` text COLLATE utf8mb4_bin NOT NULL,
  `ru` text COLLATE utf8mb4_bin NOT NULL