Check to see if a value is NULL.
$value: The value on which the assertion is to be done.
$message: The message to display along with the assertion.
$group: The type of assertion - examples are "Browser", "PHP".
TRUE if the assertion succeeded, FALSE otherwise.
protected function assertNull($value, $message = '', $group = 'Other') {
return $this
->assert(!isset($value), $message ? $message : t('Value @value is NULL.', array(
'@value' => var_export($value, TRUE),
)), $group);
}