Is it possible to replace the NULL value of a database with the current year in php pdo?

I am new to PHP / MySQL PDO. I would like to know if it was possible to change the "NULL" value of a database to do calculations without modifying the NULL in the database.

In my database, the "NULL" correspond to the current year (this is a list of prime ministers who have either not completed their term or are not dead).

I need to create a form that searches for the list of prime ministers mandating by mandate date.

I would like to replace the "NULL" of the database by a variable taking the current year generated by $var = date('Y'); in my PHP code.

Screenshot of the database

It’s not easy to explain. I would like the "NULL" to stay "NULL" in the database.

In my PHP code, I would like to assign a value to these NULLs without changing the structure in the database so that I can perform operations.

I created a form that displays by year all the ministers of Belgium mandating that year. I use the "NULL" in the database to say that the minister is not yet deceased or has not yet completed his term. To display the minister of the year 2020 (and who has not yet finished his mandate), I need to say that the "NULL" of the database is equivalent to the current year because we do not know predict whether the minister will resign or will have completed his term as long as it is not changed in the official year in the database.

I was wondering if it was possible to say (in PHP) that "NULL" are the current year, like for example $ var = date (‘Y’); but saying "NULL" equals date (‘Y’).

If this is not possible, how could I update the NULL data in PHP prepared request in the database based on the current year? Something like UPDATE table SET nom_colonne_1 = 'nouvelle valeur' WHERE condition

My current code to search by mandate year:

<form method="GET" action="index.php">
  <label for="annee">Année de mandat: </label>
  <input type="number" name="annee">
  <input type="submit" name="rechercher" value="Rechercher">
</form>

<?php
  date_default_timezone_set('Europe/Brussels');
  $creation_belgique = 1831;
  $aujourdhui = date('Y'); // Y = reprend l'année en cours automatiquement
  if(isset($_GET['rechercher'])) { // Rechercher les premiers ministres mandatant en fonction de l'année choisie du formulaire (du début à la fin du mandat)
    $req_recherche=$bdd->prepare("SELECT * FROM premiers INNER JOIN partis ON premiers.id_partis=partis.id_partis WHERE annee_debut<=:annee AND annee_fin>=:annee"); // préformation de la requête au serveur 
    $req_recherche->execute(array('annee'=>$_GET['annee']));

    // Rechercher le roi régnant en fonction de l'année choisie du formulaire (du début à la fin du règne)
    $req_recherche_b=$bdd->prepare("SELECT * FROM rois WHERE annee_debut<=:annee AND annee_fin>=:annee"); // préformation de la requête au serveur 
    $req_recherche_b->execute(array('annee'=>$_GET['annee']));
    $rois=$req_recherche_b->fetch();

    // Si l'année est inférieure à la date de la création de la Belgique -> pas de premier ministre
    if ($_GET['annee'] < $creation_belgique) {
       echo "<p>Avant 1831, date de création de la Belgique, il n'y avait pas de Premier ministre.";
    } else {  // Si l'année correspond à une date de mandat -> affichage des premiers ministres
       echo '<p>En ',$_GET['annee'],', sous le règne de ', 
       $rois['nom_rois'],', les Premiers ministres mandatant sont :';

       while($premiers_annee=$req_recherche->fetch()) {
           echo '<li>',$premiers_annee['nom_premiers'],', ',$premiers_annee['annee_debut'],', ',$premiers_annee['nom_partis'],'<br>';
       }
       $req_recherche->closeCursor();
       $req_recherche_b->closeCursor();
    }
}
?>

Thank you for your answers

SELECT * FROM mytable 
WHERE **COALESCE**(column_with_nulls, extract('year' from CURRENT_DATE)) BETWEEN param1 AND param2
`

CREATE TABLE IF NOT EXISTS `db_table` (
  `id` int(32) unsigned NOT NULL,
  `datas` int(32) unsigned,
  PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `db_table` (`id`, `datas`) VALUES (1, 3), (2, null), (3, 11), (4, null);

SELECT id, IF(datas IS NULL, NOW(), datas)
FROM db_table