Fix activation crash, allow disabling founder flag

Fix activation crash, allow disabling founder flag

Error was:
> Uncaught PDOException: SQLSTATE[22007]:
> Invalid datetime format: 1366 Incorrect integer value: '' for column `cweiske-gamestick`.`users`.`founderFlag` at row 1
> in /home/cweiske/www/gamestick.cweiske.de/pjgsapi/src/UserRepository.php:63
>
> Stack trace:
> #0 /home/cweiske/www/gamestick.cweiske.de/pjgsapi/src/UserRepository.php(63): PDOStatement->execute()
> #1 /home/cweiske/www/gamestick.cweiske.de/pjgsapi/src/User.php(66): UserRepository->update()
> #2 /home/cweiske/www/gamestick.cweiske.de/pjgsapi/www/activate.php(79): User->update()
> #3 {main}
>
> thrown in /home/cweiske/www/gamestick.cweiske.de/pjgsapi/src/UserRepository.php on line 63