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 ]