Der går du... ved at bruge en afledt tabel.
SELECT * FROM
(
SELECT
[TableName] = so.name,
[RowCount] = MAX(si.rows)
FROM
sysobjects so,
sysindexes si
WHERE
so.xtype = 'U'
AND
si.id = OBJECT_ID(so.name)
GROUP BY
so.name
) sub
WHERE sub.[RowCount] = 0