Fix JsonRecordPacker when bytes field is None

[yunzheng]

Fixes #179

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.98%. Comparing base (820113c) to head (9ecd867).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #180   +/-   ##
=======================================
  Coverage   82.97%   82.98%           
=======================================
  Files          34       34           
  Lines        3601     3602    +1     
=======================================
+ Hits         2988     2989    +1     
  Misses        613      613           

Flag	Coverage Δ
unittests	82.98% <100.00%> (+<0.01%)	⬆️

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[Schamper]

This change seems irrelevant?

[yunzheng]

Yes, I noticed a deprecation warning during pytest, I was lazy to create a dedicated PR :D

I can still move it to another PR if you think that is better.