Enforced parameter and return value types in Driver classes

[morozov]

Q	A
Type	improvement
BC Break	yes

The rationale behind the breaking changes is the following:

1. Most of the underlying drivers do not allow NULL for $username or $password, therefore if we allow NULL, we'll have to replace it with an empty string in most of the implementation. At the same time, fro the drivers which support NULL (e.g. PDO) there's no difference between NULL and empty string.
2. Some drivers (e.g. mysqli, pdo_mysql, pdo_sqlite) allow establishing a connection without selecting a database, therefore it should be possible to indicate the absence of a value in getDatabase(). This is important because consumers (mostly, schema managers) should handle the situation of "no database selected" (which they currently don't) and raise an error instead of returning an empty list of assets.

[morozov]

Thank you @greg0ire. I wish I could use Grammarly with PhpStorm.

[jwage]

Hooray!