Check to see if a value is NULL.
$value: The value on which the assertion is to be done.
$message: (optional) A message to display with the assertion. Do not translate messages: use format_string() to embed variables in the message text, not t(). If left blank, a default message will be displayed.
$group: (optional) The group this message is in, which is displayed in a column in test output. Use 'Debug' to indicate this is debugging output. Do not translate this string. Defaults to 'Other'; most tests do not override this default.
TRUE if the assertion succeeded, FALSE otherwise.
protected function assertNull($value, $message = '', $group = 'Other') {
return $this
->assert(!isset($value), $message ? $message : t('Value @value is NULL.', array(
'@value' => var_export($value, TRUE),
)), $group);
}