Fix #67131: setcookie() conditional for empty values not met

Fix #67131: setcookie() conditional for empty values not met

PHP applies a workaround for old MSIE where setting an empty cookie value would
not delete the cookie. This workaround is only triggered if an empty string (or
a value that converts to an empty string) is actually given as $value parameter
of setcookie. If the $value parameter is omitted, an empty cookie value is
sent. This commit fixes the inconsistent behavior.