TAJO-881: JOIN with union query occurs NPE

Join with tableC and result of union tableA, tableB is expected the following physical plan. But Union execution block is not necessary.
```
       |-eb_0001_000006 (Terminal)
          |-eb_0001_000005 (Join eb_0001_000003, eb_0001_000004)
             |-eb_0001_000004 (Scan TableC)
             |-eb_0001_000003 (Union TableA, TableB)
               |-eb_0001_000002 (Scan TableB)
               |-eb_0001_000001 (Scan TableA)
```
The above plan can be changed to the following plan.
``` 
      |-eb_0001_000005 (Terminal)
          |-eb_0001_000003    (Join [eb_0001_000001, eb_0001_000002], eb_0001_000004)
             |-eb_0001_000004 (Scan TableC)
             |-eb_0001_000002 (Scan TableB)
             |-eb_0001_000001 (Scan TableA)
```
eb_0001_000003's left child should be eb_0001_000001 + eb_0001_000001 and right child should be eb_0001_000004.
For this eb_0001_000001 is representative of eb_0001_000001, eb_0001_000002.
So eb_0001_000003's left child is eb_0001_000001