add missing exists method to the null driver.

fixes:
PHP Fatal error:  Call to undefined method Horde_Core_Auth_Signup_Null::exists() in /home/mcr/Entwicklung/horde/framework/Core/lib/Horde/Core/Auth/Signup/Base.php on line 131, referer: http://localhost/signup.php
PHP Stack trace:, referer: http://localhost/signup.php
PHP   1. {main}() /home/mcr/Entwicklung/horde/horde/signup.php:0, referer: http://localhost/signup.php
PHP   2. Horde_Core_Auth_Signup_Base->addSignup($info = *uninitialized*) /home/mcr/Entwicklung/horde/horde/signup.php:46, referer: http://localhost/signup.php
PHP   3. Horde_Core_Auth_Signup_Base->_preSignup($info = *uninitialized*) /home/mcr/Entwicklung/horde/framework/Core/lib/Horde/Core/Auth/Signup/Base.php:31, referer: http://localhost/signup.php