Make sum-of-empty-set behavior configurable (NULL vs 0)

[kadinrabo]

Currently, sum of zero elements returns NULL, matching the SQL standard and most
databases (PostgreSQL, DuckDB, Spark, Trino, SQLite).

However, some engines like ClickHouse return 0 for sum of an empty set:

docker run clickhouse/clickhouse-server:latest clickhouse-local \
  --query "SELECT SUM(x) FROM (SELECT 1 AS x WHERE false) t"

We should consider adding an option to make this behavior configurable.

Context:

• feat(extensions): add unsigned integer extension types (u8, u16, u32, u64) #953 (comment)
• feat(extensions): add unsigned integer extension types (u8, u16, u32, u64) #953 (comment)