rbindlist(..., fill=TRUE) fills missing list columns with NA instead of NULL

[sritchie73]

Fix for #4198 , if desired.

Current behaviour:

> A = data.table(c1=0L, c2=list(1:3))
> B = data.table(c1=1:2)
> rbind(A,B,fill=TRUE)
      c1     c2
   <num> <list>
1:     0  1,2,3
2:     1   
3:     2

New behaviour:

> rbind(A,B,fill=TRUE)
      c1     c2
   <int> <list>
1:     0  1,2,3
2:     1     NA
3:     2     NA

[jangorecki]

setting WIP till we agree on expected behaviour in related issue: #4198 (comment)

[codecov]

Codecov Report

Merging #4199 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4199      +/-   ##
==========================================
+ Coverage   99.61%   99.61%   +<.01%     
==========================================
  Files          72       72              
  Lines       13871    13876       +5     
==========================================
+ Hits        13817    13822       +5     
  Misses         54       54

Impacted Files	Coverage Δ
src/rbindlist.c	100% <100%> (ø)	⬆️

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