fix(physical-plan): count empty grouping sets in the aggregate row estimate for an empty input

[asolimando]

Which issue does this PR close?

• Closes COUNT(*) over an empty grouping set with empty input returns 0 (should return 1) #24038.
• Part of Epic: Statistics improvements #8227.

Rationale for this change

AggregateExec reported a row count of Exact(0) whenever its input had no rows, even when the
group-by contained an empty grouping set. GROUPING SETS(()), ROLLUP and CUBE each emit a
grand-total row for such an input, so the row count contradicted execution. The count was claimed
as Exact, so rules that answer a query from statistics rather than by executing it returned a
value that no row of the result holds:

SELECT COUNT(*) FROM (SELECT SUM(v1) FROM t WHERE false GROUP BY ROLLUP(v1));
-- 0, while the inner aggregate emits one row

This is a correctness fix, not an estimation improvement.

What changes are included in this PR?

• estimate_num_rows counts the empty grouping sets instead of reusing the child's row count of
  zero. Partial aggregation emits the grand-total row from every output partition, so the count
  goes through the per-partition scaling already applied to aggregates without grouping
  expressions, now factored into scale_logical_rows.
• The grouping columns of such an output are reported as NULL: typed null bounds, a distinct
  count of zero, and a null count equal to the row count.

Are these changes tested?

Yes.

• SQL logic tests in grouping.slt for both folds: an outer COUNT(*) over ROLLUP, CUBE and
  GROUPING SETS, and an outer MIN/MAX over the grand-total row.
• Unit tests for the reported statistics: zero rows for a plain GROUP BY, one row for
  GROUPING SETS((a), ()), two rows for GROUPING SETS((a), (), ()), the partition scaling of a
  partial aggregate, and the NULL grouping-column statistics.

Each test was checked to fail without the corresponding change.

Are there any user-facing changes?

Yes. Queries such as the one above now return the correct result. There are no API changes.

---

Disclaimer: I used AI to assist in the code generation, I have manually reviewed the output and it matches my intention and understanding.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.41176% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.86%. Comparing base (dbcb5c0) to head (42161dc).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/aggregates/mod.rs	89.41%	5 Missing and 13 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24039    +/-   ##
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  Coverage   80.85%   80.86%            
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  Files        1101     1101            
  Lines      374933   375633   +700     
  Branches   374933   375633   +700     
========================================
+ Hits       303166   303746   +580     
- Misses      53671    53775   +104     
- Partials    18096    18112    +16     

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