GROUPING SETS (()) returns no rows on empty input

[xiedeyantu]

Describe the bug

When a query uses GROUP BY GROUPING SETS(()) and the input relation is empty, DataFusion returns 0 rows. This is incorrect.
According to SQL semantics, the empty grouping set () represents a global aggregate, so it should still produce exactly one output row even when there are no input rows.

In practice, this affects queries such as:

SELECT SUM(empno)
FROM emp
WHERE false
GROUP BY GROUPING SETS(());

The current result is an empty result set, but it should return one row with NULL for SUM(empno).

To Reproduce

Steps to reproduce the behavior:

Create a table and insert data.
Run a query with WHERE false so the input becomes empty.
Use GROUP BY GROUPING SETS(()).
Example:

CREATE TABLE emp (
  empno INT PRIMARY KEY,
  ename VARCHAR(10) NOT NULL,
  job VARCHAR(10),
  mgr INT,
  hiredate DATE,
  sal DECIMAL(10,2),
  comm DECIMAL(10,2),
  deptno INT
);

INSERT INTO emp (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO) VALUES
(7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800.00, NULL, 20);

SELECT SUM(empno)
FROM emp
WHERE false
GROUP BY GROUPING SETS(());

Returns:

+----------------+
| sum(emp.empno) |
+----------------+
+----------------+
0 row(s) fetched.

Expected behavior

Returns:

+----------------+
| sum(emp.empno) |
+----------------+
| NULL           |
+----------------+
1 row(s) fetched.

Additional context

No response

[xiedeyantu]

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