[R-Forge #5582] When DT is empty, unique(DT, by=key(DT)) returns a data.table with one row, all NAs

Submitted by: shubh bansal; Assigned to: Arun ; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5582&group_id=240&atid=975)

``` S
dt=data.table(a=character(0), b=numeric(0), key='a')
unique(dt)
#     a  b
#1: NA NA
```
