Add aggregate_functions_null_for_empty setting for SQL standard compatibility

Why it matters

This feature addresses the discrepancy between ClickHouse aggregate functions and the SQL standard by making aggregate functions return null for empty input sets, improving query consistency and compatibility with SQL expectations.

How to use it

Enable the feature by setting aggregate_functions_null_for_empty = 1 in the session or query settings. This will automatically rewrite aggregate functions to their -OrNull variants during query execution.