executor, plan: eliminate projection if the schema len is 0

[XuHuaiyu]

Before this commit:

tidb(127.0.0.1)> create table t(a int);
Query OK, 0 rows affected (0.11 sec)

tidb(127.0.0.1)> insert into t value(0),(0);
Query OK, 2 rows affected (0.00 sec)

tidb(127.0.0.1)> select 1 from (select a from t union all select a from t) tmp;
0 rows in set (0.00 sec)

tidb(127.0.0.1)> explain select 1 from (select a from t union all select a from t) tmp;
+----------------+---------------+----------------------------+------+----------------------------------------------+----------+
| id             | parents       | children                   | task | operator info                                | count    |
+----------------+---------------+----------------------------+------+----------------------------------------------+----------+
| TableScan_10   |               |                            | cop  | table:t, range:[-inf,+inf], keep order:false | 10000.00 |
| TableReader_11 | Projection_9  |                            | root | data:TableScan_10                            | 10000.00 |
| Projection_9   | Union_8       | TableReader_11             | root |                                              | 10000.00 |
| TableScan_13   |               |                            | cop  | table:t, range:[-inf,+inf], keep order:false | 10000.00 |
| TableReader_14 | Projection_12 |                            | root | data:TableScan_13                            | 10000.00 |
| Projection_12  | Union_8       | TableReader_14             | root |                                              | 10000.00 |
| Union_8        | Projection_7  | Projection_9,Projection_12 | root |                                              | 20000.00 |
| Projection_7   |               | Union_8                    | root | 1                                            | 20000.00 |
+----------------+---------------+----------------------------+------+----------------------------------------------+----------+
8 rows in set (0.00 sec)

Which is caused by the columnPruner pruning all the columns of Projection,
thus there is no data returned from Projection.

After this commit,

tidb(127.0.0.1)> select 1 from (select a from t union all select a from t) tmp;

+---+
| 1 |
+---+
| 1 |
| 1 |
| 1 |
| 1 |
+---+

tidb(127.0.0.1)> explain select 1 from (select a from t union all select a from t) tmp;
+----------------+--------------+-------------------------------+------+----------------------------------------------+----------+
| id             | parents      | children                      | task | operator info                                | count    |
+----------------+--------------+-------------------------------+------+----------------------------------------------+----------+
| TableScan_10   |              |                               | cop  | table:t, range:[-inf,+inf], keep order:false | 10000.00 |
| TableReader_11 | Union_8      |                               | root | data:TableScan_10                            | 10000.00 |
| TableScan_13   |              |                               | cop  | table:t, range:[-inf,+inf], keep order:false | 10000.00 |
| TableReader_14 | Union_8      |                               | root | data:TableScan_13                            | 10000.00 |
| Union_8        | Projection_7 | TableReader_11,TableReader_14 | root |                                              | 20000.00 |
| Projection_7   |              | Union_8                       | root | 1                                            | 20000.00 |
+----------------+--------------+-------------------------------+------+----------------------------------------------+----------+
6 rows in set (0.01 sec)

PTAL @zz-jason @winoros

[zz-jason]

/run-all-tests

[XuHuaiyu]

/run-common-test
/run-integration-common-tests

[XuHuaiyu]

/run-common-test tidb-test=pr/504
/run-integration-common-tests tidb-test=pr/504

[zz-jason]

LGTM

[winoros]

LGTM
though i think it should handled more elegantly