[5.5] return empty string if database session key was expired

[themsaid]

To be consistent with the SessionHandler interface:

	/**
	 * Read session data
	 * @link http://php.net/manual/en/sessionhandlerinterface.read.php
	 * @param string $session_id The session id to read data for.
	 * @return string <p>
	 * Returns an encoded string of the read data.
	 * If nothing was read, it must return an empty string.
	 * Note this value is returned internally to PHP for processing.
	 * </p>
	 * @since 5.4.0
	 */
	public function read($session_id);