Runtime: sum/avg over empty input, avg overflow, and unsupported SQL shapes #659

Found by the DataFusion differential check of #539, executing lowered SQL plans with the #541 runtime and comparing with DataFusion 54.

Wrong results

• sum over empty or all-NULL input returns 0, not NULL (e.g. SELECT sum(a) FROM empty_table, or a join group whose values are all NULL). avg returns NaN instead of NULL.
• avg of an Int64 column accumulates in a wrapping 64-bit integer: avg over i64::MAX, i64::MIN, -7 gives -2.67.

Differences

• 64-bit +, -, *, unary minus and i64::MIN % -1 raise an overflow error; DataFusion wraps around (and returns 0 for the modulo).

Not supported by the runtime

Set operations, Concat (grouping sets), subquery scalars, IN lists, LIKE / ILIKE, casts from float or string to integer, upper / round / nullif, filtered aggregates (including count of a nullable column), the count-distinct / stddev / variance / quantile / corr / argMax aggregates, window functions, ORDER BY on a non-projected expression, and the TopK aggregate. About 40% of the 573 checked queries could not run for these reasons.

Possible fix

Return NULL from sum / avg over no non-NULL input; accumulate avg in a float or wide integer; decide whether integer overflow follows DataFusion (wrap) or errors, and document it.