[R-Forge #5405] unqiue fails when data.table is NULL

Submitted by: agstudy agstudy; Assigned to: Arun ; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5405&group_id=240&atid=975)

Amazing but this 

```
unique(data.table(NULL))
```

returns an error:

Error in duplicated.data.table(x, incomparables, tolerance, by, ...) : 
  invalid subscript type 'list'

should return NULL data.table.
