GROUP BY ROLLUP returns 0 rows with empty input (should return grand total row)

**Describe the bug**

When `GROUP BY ROLLUP` is used with a query that produces 0 input rows, DataFusion returns 0 output rows. The SQL standard requires ROLLUP to always produce the grand total row (with NULL grouping keys and aggregate results computed over the empty set).

**To Reproduce**

```sql
CREATE TABLE test (category VARCHAR, value INT) AS VALUES ('a', 1);

-- With data: works correctly (returns 2 rows: group + grand total)
SELECT category, count(*) FROM test GROUP BY ROLLUP(category);
-- Returns: [('a', 1), (NULL, 1)] ✅ correct

-- With empty input: returns 0 rows instead of 1
SELECT category, count(*) FROM test WHERE value < 0 GROUP BY ROLLUP(category);
-- Returns: [] ❌ should return [(NULL, 0)]

-- Without GROUP BY, empty input correctly returns 1 row:
SELECT count(*) FROM test WHERE value < 0;
-- Returns: [(0)] ✅ correct
```

**Expected behavior**

Per SQL standard (and PostgreSQL, Trino, Oracle behavior):

```sql
SELECT category, count(*) FROM test WHERE value < 0 GROUP BY ROLLUP(category);
-- Should return: [(NULL, 0)]
```

The grand total grouping set (the empty grouping set that ROLLUP always includes) should produce a row even when there are zero input rows, because aggregate functions like `count(*)` have well-defined behavior on empty sets (returns 0).

**Additional context**

- Tested on DataFusion 52.3.0
- Same behavior with `CUBE` and explicit `GROUPING SETS((), (category))`
- Regular `GROUP BY category` correctly returns 0 rows with empty input (no grand total expected)
- This affects 6 TPC-DS queries (q18, q27, q36, q67, q70, q86) at small scale factors where certain joins produce 0 matching rows

**Root cause (likely)**

DataFusion's grouped aggregation implementation skips producing output batches when the hash table is empty (no input rows). For regular `GROUP BY`, this is correct. For `ROLLUP`/`CUBE`/`GROUPING SETS` that include the empty grouping set `()`, a row should always be produced for that set.

The fix would need to check if any grouping set is the empty set, and if so, produce a row with NULL grouping keys and empty-set aggregate values (count=0, sum=NULL, etc.) even when the hash table has no entries.

The issue names no files or tests. Start by locating DataFusion's grouped aggregation implementation and grouping-set handling, then compare empty-input behavior with regular GROUP BY. Add a regression test covering empty-input ROLLUP, CUBE, or GROUPING SETS with count, and verify the grand-total row is returned.

Written by the indexing model from the issue text.