plan: statsInfo.count for LogicalTableDual should be its RowCount

[zz-jason]

Implement deriveStats for LogicalTableDualto make statsInfo.count be its RowCount.
Before this PR:

TiDB(localhost) > desc select * from t where 1 = 0;
+--------------+--------------+-------------+------+--------------------+-------+
| id           | parents      | children    | task | operator info      | count |
+--------------+--------------+-------------+------+--------------------+-------+
| TableDual_6  | Projection_5 |             | root | rows:0             | 1.00  |
| Projection_5 |              | TableDual_6 | root | test.t.a, test.t.b | 1.00  |
+--------------+--------------+-------------+------+--------------------+-------+
2 rows in set (0.00 sec)

After this PR:

TiDB(localhost) > desc select * from t where 1 = 0;
+--------------+--------------+-------------+------+--------------------+-------+
| id           | parents      | children    | task | operator info      | count |
+--------------+--------------+-------------+------+--------------------+-------+
| TableDual_6  | Projection_5 |             | root | rows:0             | 0.00  |
| Projection_5 |              | TableDual_6 | root | test.t.a, test.t.b | 0.00  |
+--------------+--------------+-------------+------+--------------------+-------+
2 rows in set (0.00 sec)

[zz-jason]

/run-all-tests

[winoros]

LGTM

[zimulala]

LGTM