empty argument in j output caught early & stopped

[MichaelChirico]

Closes #3507 Given that some other languages support trailing commas, e.g. Python:

python3
$ x = [1, 2, ]
$ x
# [1, 2]

It might be worth considering whether to support that here by simply dropping that element of jsub (which would break from base R).

Probably coders should be held to picking their commas to match the language -- in R, it's better to use leading commas rather than trailing (for the record, this practice greatly facilitates including/dropping fields in exploratory work, since lines can be commented/uncommented without breaking syntax)

[codecov]

Codecov Report

Merging #3510 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3510      +/-   ##
==========================================
+ Coverage   96.68%   96.68%   +<.01%     
==========================================
  Files          65       65              
  Lines       12280    12281       +1     
==========================================
+ Hits        11873    11874       +1     
  Misses        407      407

Impacted Files	Coverage Δ
R/data.table.R	97.64% <100%> (ø)	⬆️

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[mattdowle]

Great. For completeness, done : 8eb3bf4