unwrap j=list(var) for constants

[ben-schwen]

Closes #712
Should probably follow #7401

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.96%. Comparing base (851467f) to head (2311646).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7565   +/-   ##
=======================================
  Coverage   98.96%   98.96%           
=======================================
  Files          87       87           
  Lines       16734    16737    +3     
=======================================
+ Hits        16560    16563    +3     
  Misses        174      174           

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[github-actions]

• HEAD=optimize_listVar stopped early for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 2311646

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

Task	Duration
R setup and installing dependencies	5 minutes and 7 seconds
Installing different package versions	11 minutes and 24 seconds
Running and plotting the test cases	4 minutes and 5 seconds