Fix - Validation for mandatory multiple dropdown (#937)

* Fix validation for mandatory multiple dropdown

* Update CHANGELOG

* Update inc/container.class.php

Co-authored-by: Romain B. <8530352+Rom1-B@users.noreply.github.com>

---------

Co-authored-by: Romain B. <8530352+Rom1-B@users.noreply.github.com>

Author: RomainLvr

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [UNRELEASE]
 
+### Fixed
+
+- Fix validation for mandatory multiple dropdown
+
 ## [1.21.21] - 2025-03-21
 
 ### Fixed

inc/container.class.php

@@ -1521,8 +1521,7 @@ public static function validateValues($data, $itemtype, $massiveaction)
             if (
                 $field['mandatory'] == 1
                 && (
-                    $value === null
-                    || $value === ''
+                    empty($value)
                     || (($field['type'] === 'dropdown' || preg_match('/^dropdown-.+/i', $field['type'])) && $value == 0)
                     || (in_array($field['type'], ['date', 'datetime']) && $value == 'NULL')
                 )