.SDcols gives a strange message if length() 0

[MichaelChirico]

Also identified in a comment by @HughParsonage here:

#1789 (comment)

For now I'll solve this by just short-circuiting to null.data.table() if !length(.SDcols)... the broader issue raised in #1789 of handling length-0 .SDcols in the case of := will require a bit more work.