Null-able ProducerRecord and RecordMetadata members

[bobby]

Checklist

• [x] tests
• [x] updated CHANGELOG (the "unreleased" section)

[codecov[bot]]

Codecov Report

Merging #220 into master will decrease coverage by 0.38%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
- Coverage   79.97%   79.59%   -0.39%     
==========================================
  Files          42       42              
  Lines        2597     2597              
  Branches      150      150              
==========================================
- Hits         2077     2067      -10     
- Misses        370      380      +10     
  Partials      150      150

Impacted Files	Coverage Δ
src/jackdaw/serdes/edn2.clj	45.45% <0%> (-45.46%)	:arrow_down:

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[bobby]

@cddr I've added a few tests to exercise these fixes.

[cddr]

@bobby Awesome thanks for that. LGTM :-)