Replace ISNULL with IS NULL

Using the isnull function is also safe to use but is not semantically
correct to use. This function is used in select statements to format the
selected data based on the value of the column that is selected.

Author: MKodde

library/EngineBlock/Corto/Model/Consent.php

@@ -194,7 +194,7 @@ private function _hasStoredConsent(ServiceProvider $serviceProvider, $consentTyp
                   AND service_id = ?
                   AND attribute = ?
                   AND consent_type = ?
-                  AND ISNULL(deleted_at)
+                  AND deleted_at IS NULL
             ";
             $hashedUserId = sha1($this->_getConsentUid());
             $parameters = array(

library/EngineBlock/User.php

@@ -60,7 +60,7 @@ public function getConsent()
         $pdo = $this->_getDatabaseConnection();
         $query = 'SELECT service_id FROM consent
                     WHERE hashed_user_id = ?
-                    AND ISNULL(deleted_at)';
+                    AND deleted_at IS NULL';
         $parameters = array(
             sha1($this->getUid())
         );

src/OpenConext/EngineBlockBundle/Authentication/Repository/DbalConsentRepository.php

@@ -67,7 +67,7 @@ public function findAllFor($userId)
             WHERE
                 hashed_user_id=:hashed_user_id
             AND
-                ISNULL(deleted_at)
+                deleted_at IS NULL
         ';
 
         try {
@@ -129,7 +129,7 @@ public function deleteOneFor(string $userId, string $serviceProviderEntityId): b
                 hashed_user_id = :hashed_user_id
             AND
                 service_id = :service_id
-            AND ISNULL(deleted_at)
+            AND deleted_at IS NULL
         ';
         try {
             $result = $this->connection->executeQuery(