Return `null` when applicable from `getGeneratedKeys`

[elpete]

Currently, there is no way to check if getGeneratedKeys returned null. Instead, when trying to call ResultSet.next() a NullPointerException is thrown.

This method should return an empty ResultSet when there are no generated keys, but not all JDBC drivers do (like Apache Derby) In these cases, I think the safest option is to return null, which can then be checked (and is checked in popular JDBC libraries like JOOQ.)