Honor stringtype=unspecified when also saving null values

This change applies the `stringtype=unspecified` setting to null values when using JDBC's `setNull` method with a sqlType of `VARCHAR` in addition to the `setString` method.

Currently, saving a string with `setString` applies `oid.UNSPECIFIED`, but saving null with `setNull(index, java.sql.Types.VARCHAR)` saves with `oid.VARCHAR`.

This causes problem with ORM layers that are, for example, saving strings to timestamp columns, which works in the non-null case, but creates a postgresql error when null.