Add Aggregate Function Combinators -ornull and -ordefault, Which Return Null or Default Values When There Is Nothing to Aggregate

Why it matters

These combinators solve the problem of aggregate functions returning no result (empty aggregation), providing a way to output null or a specified default value instead. This improves query robustness and makes handling empty aggregations more convenient.

How to use it

Use the new combinators by appending -OrNull or -OrDefault to existing aggregate function names. For example, count-OrNull() returns null when no rows match, and sum-OrDefault() returns zero or the aggregate's default value in that case.