Fix json_read crashing when the new population is empty

[dpaetzel]

I.e. when the JSON file only contains { "classifiers" : [] }. This so far always resulted in a segfault which can't be desired.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.59%. Comparing base (bf87a28) to head (7f3f7e6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #205   +/-   ##
=======================================
  Coverage   70.58%   70.59%           
=======================================
  Files          61       61           
  Lines        8939     8940    +1     
  Branches      933      934    +1     
=======================================
+ Hits         6310     6311    +1     
  Misses       2629     2629           

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[dpaetzel]

The jobs fail probably because of the C tests, I'll check, sorry

[rpreen]

Thanks for this fix.