fix device selection logic

[StRigaud]

return null if no device found but backend found

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 11.76471% with 15 lines in your changes missing coverage. Please review.

Project coverage is 46.65%. Comparing base (a733d25) to head (86bfeaa).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
clic/src/openclbackend.cpp	11.76%	12 Missing and 3 partials ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #402   +/-   ##
=======================================
  Coverage   46.65%   46.65%           
=======================================
  Files         199      199           
  Lines        3972     3972           
  Branches     1815     1813    -2     
=======================================
  Hits         1853     1853           
+ Misses        714      713    -1     
- Partials     1405     1406    +1     

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