fix: merge into panic

[zhyass]

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Summary

The reason is that the condition is false, after RuleEliminateFilter, it is converted to ConstantTableScan, which makes the fileds in mutation invalid.

mysql> create table t(a int);
Query OK, 0 rows affected (0.05 sec)

mysql> create table s(a int);
Query OK, 0 rows affected (0.05 sec)

mysql> merge into s using t on 1 != 1 when matched and s.a>0 then update * when not matched then insert *;
+-------------------------+------------------------+
| number of rows inserted | number of rows updated |
+-------------------------+------------------------+
|                       0 |                      0 |
+-------------------------+------------------------+
1 row in set (0.15 sec)
Read 0 rows, 0.00 B in 0.067 sec., 0 rows/sec., 0.00 B/sec.

• fixes: bug: merge into panic if condition is false #16588

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[wubx]

merge into s using t on 1 != 1 when matched and s.a>0 then update * when not matched then insert *;

This SQL statement may cause unexpected behavior. Is it okay to make the error reports more user-friendly?