Fix trivial log issue

[kangkaisen]

Currently, explain insert into t1 SelectStmt output is empty.

After fixing this bug, explain insert into t1 select * from t2; output is following:

+----------------------------------------------------------------------------------------------------------------------+
| Explain String                                                                                                       |
+----------------------------------------------------------------------------------------------------------------------+
| PLAN FRAGMENT 0                                                                                                      |
|  OUTPUT EXPRS:`default_cluster:test.t2`.`dt` | `default_cluster:test.t2`.`id` | `default_cluster:test.t2`.`value` |  |
|   PARTITION: RANGE_PARTITIONED: <slot 0>                                                                             |
|                                                                                                                      |
|   DPP SINK                                                                                                           |
|     TABLE NAME: t1                                                                                                   |
|     PARTITIONS: 10008, 10009, 10010                                                                                  |
|                                                                                                                      |
|   2:EXCHANGE                                                                                                         |
|      tuple ids: 0                                                                                                    |
|                                                                                                                      |
| PLAN FRAGMENT 1                                                                                                      |
|  OUTPUT EXPRS:                                                                                                       |
|   PARTITION: RANDOM                                                                                                  |
|                                                                                                                      |
|   STREAM DATA SINK                                                                                                   |
|     EXCHANGE ID: 02                                                                                                  |
|     RANGE_PARTITIONED: <slot 0>                                                                                      |
|                                                                                                                      |
|   1:OLAP REWRITE NODE                                                                                                |
|   |  tuple ids: 0                                                                                                    |
|   |                                                                                                                  |
|   0:OlapScanNode                                                                                                     |
|      TABLE: t2                                                                                                       |
|      PREAGGREGATION: ON                                                                                              |
|      partitions=1/1                                                                                                  |
|      rollup: t2                                                                                                      |
|      buckets=10/10                                                                                                   |
|      cardinality=2                                                                                                   |
|      avgRowSize=1889.4286                                                                                            |
|      tuple ids: 1                                                                                                    |
+----------------------------------------------------------------------------------------------------------------------+

[kangkaisen]

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