retain unused levels from zero-row data.table in rbindlist

[mattdowle]

Closes #3508

[codecov]

Codecov Report

Merging #3586 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3586      +/-   ##
==========================================
+ Coverage   97.63%   97.63%   +<.01%     
==========================================
  Files          66       66              
  Lines       12701    12702       +1     
==========================================
+ Hits        12400    12401       +1     
  Misses        301      301

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

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[codecov]

Codecov Report

Merging #3586 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3586      +/-   ##
==========================================
+ Coverage   97.63%   97.63%   +<.01%     
==========================================
  Files          66       66              
  Lines       12701    12702       +1     
==========================================
+ Hits        12400    12401       +1     
  Misses        301      301

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d7d5df4...69f4daf. Read the comment docs.