Make operator bool() explicit

[mborland]

Closes: #321

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (a18ddec) to head (14248d1).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #323   +/-   ##
========================================
  Coverage    99.02%   99.02%           
========================================
  Files           61       61           
  Lines         5318     5318           
  Branches      1643     1643           
========================================
  Hits          5266     5266           
  Misses          21       21           
  Partials        31       31           

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