PHP static analysis provide "non-empty-string" type, which is useful for most operators accepting a string. But empty string are also sometimes accepted.

https://github.com/mongodb/mongo-php-builder/pull/49#discussion_r1463897761

I reverted to the more permissive "string" in PR #50

This ticket is to revisit if we want to make static analysis more strict, adding a non-empty-string type to the operators that don't accept empty strings.