Avoid logging for missing backends

[alecandido]

Restore 95c71fa after removal in 2fdbf85 as part of #1479.

I'm still not introducing any test, since I'd need an environment without qibojit installed. I already have an attempt of doing it, cf. #1495, but it's on hold because working locally, but not in the CI (just on macOS).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.74%. Comparing base (a074d17) to head (29ccf02).
Report is 62 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1505   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files          80       80           
  Lines       11768    11768           
=======================================
  Hits        11738    11738           
  Misses         30       30           

Flag	Coverage Δ
unittests	99.74% <100.00%> (ø)

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