Support LowCardinality nullability with join_use_nulls

Why it matters

This feature addresses the lack of compatibility between LowCardinality columns with nullability and the join_use_nulls setting in JOIN queries. It enables proper handling of nullable LowCardinality keys during joins, ensuring accurate results and improved query reliability.

How to use it

To use this feature, enable the join_use_nulls setting in your JOIN queries involving LowCardinality nullable columns. For example, set the session or query option:

SET join_use_nulls = 1;

and perform the JOIN as usual. The system will correctly handle null values on LowCardinality nullable keys.