handle type coercion in fcoalesce

[ben-schwen]

Closes #7545

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (7db13b9) to head (3dbc611).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7667   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          87       87           
  Lines       17031    17044   +13     
=======================================
+ Hits        16868    16881   +13     
  Misses        163      163           

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[github-actions]

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

Generated via commit 3dbc611

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

Task	Duration
R setup and installing dependencies	3 minutes and 1 seconds
Installing different package versions	46 seconds
Running and plotting the test cases	3 minutes and 59 seconds