Phpstan thinks that SplFileObject::fgets return string but false is a possible return value

# Bug report

### Code snippet that reproduces the problem

https://phpstan.org/r/fa65d8f7-8ef3-4b99-ad70-c277ffcc3983

### Expected output

No error, false is possible according to the doc
https://www.php.net/manual/fr/function.fgets.php
