MDL-78544 forms: fix string to number comparison bug in duration element

Prior to PHP 8, 0 == "", but now no more. We should use empty instead.
See: https://www.php.net/manual/en/migration80.incompatible.php

lib/form/duration.php

@@ -157,7 +157,7 @@ class MoodleQuickForm_duration extends MoodleQuickForm_group {
      * @return array associative array ($number => $unit)
      */
     public function seconds_to_unit($seconds) {
-        if ($seconds == 0) {
+        if (empty($seconds)) {
             return [0, $this->_options['defaultunit']];
         }
         foreach ($this->get_units_used() as $unit => $notused) {