Avoid "Bad Cast" Exception for Filtering by Virtual and Index Columns in ClickHouse

Avoid "bad cast" exception when there is an expression that filters data by virtual columns (like _table in Merge tables) or by "index" columns in system tables such as filtering by database name when querying from system.tables, and this expression returns Nullable type. This fixes #12166. #12305 (alexey-milovidov).