droplevels works on 0-row tables

[MichaelChirico]

Closes #7043

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (1e2e3ba) to head (1efdaea).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7131   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files          81       81           
  Lines       15013    15013           
=======================================
  Hits        14790    14790           
  Misses        223      223           

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[github-actions]

• HEAD=drop-empty-levels slower P<0.001 for memrecycle regression fixed in #5463
• HEAD=drop-empty-levels slower P<0.001 for setDT improved in #5427
  

Generated via commit 1efdaea

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 50 seconds
Installing different package versions	9 minutes and 20 seconds
Running and plotting the test cases	2 minutes and 25 seconds