robustness -- as.data.table(table(NULL)) gives wrong error

[MichaelChirico]

as.data.table(table(NULL))
# Error in (function (..., sorted = TRUE, unique = FALSE)  : 
#   attempt to set an attribute on NULL

with full traceback():

5: (function (..., sorted = TRUE, unique = FALSE) 
   {
       l = list(...)
       if (isFALSE(getOption("datatable.CJ.names", TRUE))) {
           if (is.null(vnames <- names(l))) 
               vnames = paste0("V", seq_len(length(l)))
           else if (any(tt <- vnames == "")) 
               vnames[tt] = paste0("V", which(tt))
       }
       else {
           vnames = name_dots(...)$vnames
           if (any(tt <- vnames == "")) 
               vnames[tt] = paste0("V", which(tt))
       }
       dups = FALSE
       for (i in seq_along(l)) {
           y = l[[i]]
           if (!length(y)) 
               next
           if (sorted) {
               if (!is.atomic(y)) 
                   stop("'sorted' is TRUE but element ", i, " is non-atomic, which can't be sorted; try setting sorted = FALSE")
               o = forderv(y, retGrp = TRUE)
               thisdups = attr(o, "maxgrpn", exact = TRUE) > 1L
               if (thisdups) {
                   dups = TRUE
                   if (length(o)) 
                     l[[i]] = if (unique) 
                       y[o[attr(o, "starts", exact = TRUE)]]
                     else y[o]
                   else if (unique) 
                     l[[i]] = y[attr(o, "starts", exact = TRUE)]
               }
               else {
                   if (length(o)) 
                     l[[i]] = y[o]
               }
           }
           else {
               if (unique) 
                   l[[i]] = unique(y)
           }
       }
       nrow = prod(vapply_1i(l, length))
       if (nrow > .Machine$integer.max) 
           stop(gettextf("Cross product of elements provided to CJ() would result in %.0f rows which exceeds .Machine$integer.max == %d", 
               nrow, .Machine$integer.max, domain = "R-data.table"))
       l = .Call(Ccj, l)
       setDT(l)
       l = setalloccol(l)
       setnames(l, vnames)
       if (sorted) {
           if (!dups) 
               setattr(l, "sorted", names(l))
           else setkey(l)
       }
       l
   })(V1 = NULL, sorted = FALSE)
4: do.call(CJ, c(val, sorted = FALSE))
3: data.table(do.call(CJ, c(val, sorted = FALSE)), N = as.vector(x), 
       key = key)
2: as.data.table.table(table(NULL))
1: as.data.table(table(NULL))

Probably we should return data.table(NULL) here.

Use case: edge case of a code that was returning proper tables the rest of the time.

[MichaelChirico]

dput(as.data.frame(table(NULL)))
# structure(list(Freq = integer(0)), class = "data.frame", row.names = integer(0))

Not sure we should align to this but confirms it shouldn't error, at least