Add note about returning None

[carmocca]

What does this PR do?

Fixes #5577

To be removed with #5359

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[codecov]

Codecov Report

Merging #5578 (8c0da9b) into master (f477c2f) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5578   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         135     135           
  Lines       10005   10005           
======================================
  Hits         9339    9339           
  Misses        666     666           

[Borda]

ok, as this do not touch any code it seems that the failing tests are unrelated...