as.data.table(table(NULL)) returns null data.table

[MichaelChirico]

Closes #4179

Note the intentional break vs. base for the stated reason:

x = as.data.table(table(iris$Species))
# this implementation
rbind(x, as.data.table(table(NULL)))
#            V1     N
# 1:     setosa    50
# 2: versicolor    50
# 3:  virginica    50
# base implementation
rbind(x, as.data.frame(table(NULL)))
# Error in rbindlist(l, use.names, fill, idcol) : 
#   Item 2 has 1 columns, inconsistent with item 1 which has 2 columns. To fill missing columns use fill=TRUE.

[codecov]

Codecov Report

Merging #4180 (7d2064c) into master (dec9383) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4180   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          73       73           
  Lines       14465    14466    +1     
=======================================
+ Hits        14382    14383    +1     
  Misses         83       83           

Impacted Files	Coverage Δ
R/as.data.table.R	100.00% <100.00%> (ø)

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