Inconsistent .SDcols behavior NA vs. NA_character

Uncovered in work on #3116 

```
DT = data.table(a = 1:5)
DT[ , .SD, .SDcols = NA]
# Null data.table (0 rows and 0 cols)
DT[ , .SD, .SDcols = NA_character_]
```

> Error in `[.data.table`(DT, , .SD, .SDcols = NA_character_) : 
>   Some items of .SDcols are not column names (or are NA)

I guess should error in both cases...