CMAKE_CXX_FLAGS handling fails when variable is empty

[zaddach]

Fixes #3227

[ray6080]

Hi @zaddach , thanks for your fix!
Could you please take a look at our CLA and add “I have read and agree to the terms under CLA.md” to your PR description if you agree with the CLA?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.27%. Comparing base (d946982) to head (c7fb5a2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3228      +/-   ##
==========================================
- Coverage   92.28%   92.27%   -0.01%     
==========================================
  Files        1161     1161              
  Lines       44150    44150              
==========================================
- Hits        40744    40740       -4     
- Misses       3406     3410       +4     

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[zaddach]

I have read and agree to the terms under CLA.md