Inconsistent results in online/offline mode while join condition includes null

```
table: t1
columns: [ "c1 string","c2 int","c3 bigint","c4 timestamp" ]
indexs: [ "index1:c1:c4" ]
rows:
- [ NULL,2,3,1590738989000 ]
```
```
table: t2
- columns: [ "c1 string","c2 int","c3 bigint","c4 timestamp" ]
indexs: [ "index1:c1:c4" ]
rows:
- [ NULL,2,3,1590738989000 ]
```
`sql: select t1.c1,t1.c2,t2.c3,t2.c4 from t1 last join t2 ORDER BY t2.c4 on t1.c1=t2.c1;`
expect rows:
- [ null,2,3,1590738989000 ]

real rows:
- [ null,2,null,null ]

The issue shows a SQL join query with a NULL equality condition producing different results in online vs offline modes. Start by locating the join implementation for 'last join' in the query engine, likely in the SQL execution or optimizer modules. Examine how NULL comparisons are handled in join conditions across execution modes. Run the provided test case to reproduce the inconsistency, then trace through the join logic to identify the divergence.

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