Specify string as nullable for credentials password

Signed-off-by: Anna Larch <anna@nextcloud.com>

lib/public/Authentication/LoginCredentials/ICredentials.php

 	 *
 	 * @since 12
 	 *
-	 * @return string
+	 * @return string|null
 	 * @throws PasswordUnavailableException
 	 */
 	public function getPassword();