Empty .SDcols back to 1.11.8 behaviour, plus fix for integer()

[mattdowle]

Closes #1789
Follow up to #3185
Fixes regression in dev before release reported by Kun Ren in comments in #3211

[codecov]

Codecov Report

Merging #3238 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3238      +/-   ##
==========================================
- Coverage   94.65%   94.64%   -0.01%     
==========================================
  Files          65       65              
  Lines       12100    12099       -1     
==========================================
- Hits        11453    11451       -2     
- Misses        647      648       +1

Impacted Files	Coverage Δ
R/data.table.R	95.18% <100%> (-0.01%)	⬇️
src/uniqlist.c	95% <0%> (-0.56%)	⬇️

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[codecov]

Codecov Report

Merging #3238 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3238      +/-   ##
==========================================
- Coverage   94.65%   94.65%   -0.01%     
==========================================
  Files          65       65              
  Lines       12100    12099       -1     
==========================================
- Hits        11453    11452       -1     
  Misses        647      647

Impacted Files	Coverage Δ
R/data.table.R	95.18% <100%> (-0.01%)	⬇️

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[MichaelChirico]

always prefer to have more robustness!! thanks @kunren!!