Introduce Statement::fetchFirstColumn()

Additional changes based on the discussion in #4007

This will allow for subsequent fetches in different modes.

Psalm 6

The php docs are not very clear about that one.

It confuses both Psalm and PHPStan, and involves complicated mocking.

This return type is conditional to $params['wrapperClass']. Luckily,
recent versions of Psalm allow documenting it properly.
See https://github.com/vimeo/psalm/issues/3277

Note that phpstan is not able to understand this yet, but still attempts
to, hence the extra ignore rules.

This hack is temporary and should be removed on 3.0.x

See https://github.com/vimeo/psalm/issues/3224

This is precisely what we are testing here.

This is what is expected with FETCH_COLUMN

That alone fixes 6 issues found by Psalm

Not all RDBMS have a concept of database, apparently (see SQLAnywhere).
Those who do not should pass in a string to stay compatible with the
signature.

The method signature does not allow null.

We are testing what happens when providing the wrong type.

It is more simple and more accurate