code: protected function makeMandatory($field, $row, $key)
      {
          $field['eval']['required'] = false;
          // Use strlen() here (see contao core issue #3277).
          if (empty($field['eval']['mandatory'])) {
              return $field;
          }
  
          if (is_array($this->varValue[$row][$key])) {
              if (empty($this->varValue[$row][$key])) {
                  $field['eval']['required'] = true;
              }
          } else {
              if (!strlen($this->varValue[$row][$key])) {
                  $field['eval']['required'] = true;
              }
          }
  
          return $field;
      }
docs: Make fields mandatory if necessary.
  
  @param array  $field The field DCA.
  
  @param string $row   The setting name.
  
  @param string $key   The widget name.
  
  @return array