[SQL] [SPARK-2826] Reduce the memory copy while building the hashmap for HashOuterJoin

This is a follow up for #1147 , this PR will improve the performance about 10% - 15% in my local tests.
```
Before:
LeftOuterJoin: took 16750 ms ([3000000] records)
LeftOuterJoin: took 15179 ms ([3000000] records)
RightOuterJoin: took 15515 ms ([3000000] records)
RightOuterJoin: took 15276 ms ([3000000] records)
FullOuterJoin: took 19150 ms ([6000000] records)
FullOuterJoin: took 18935 ms ([6000000] records)

After:
LeftOuterJoin: took 15218 ms ([3000000] records)
LeftOuterJoin: took 13503 ms ([3000000] records)
RightOuterJoin: took 13663 ms ([3000000] records)
RightOuterJoin: took 14025 ms ([3000000] records)
FullOuterJoin: took 16624 ms ([6000000] records)
FullOuterJoin: took 16578 ms ([6000000] records)
```

Besides the performance improvement, I also do some clean up as suggested in #1147
This is a fairly minor nit but defining `EMPTY_LIST` instead of just using `Seq.empty[Row]`
 - Takes up extra space in the class
 - Requires double checked locking / lazy initialization logic for each access
 - Requires the developer to find the variable and figure out what it means, instead of `Seq.empty[Row]` which is standard scala.

All to save 4 characters?