uniqueN is not handling NULL value

[jangorecki]

Latest 1.9.7:

library(data.table)
uniqueN(NULL)
# Error in forderv(x, by = by, retGrp = TRUE) : 
#    First column being ordered is type 'NULL', not yet supported

[arunsrinivasan]

There's a bug when uniqueN() is used along with a list() at the moment.. it results in length(unique(.)) which would return the wrong result.

[arunsrinivasan]

This bug is as a result of #1224, not this one.