Fixes #405 - Encode nil map into null

[bbrks]

Fixes #405

• Add test to encode nil map and check for null
• Fix nil map being encoded as {}

[codecov]

Codecov Report

Merging #406 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   81.81%   81.77%   -0.05%     
==========================================
  Files          41       41              
  Lines        5022     5025       +3     
==========================================
  Hits         4109     4109              
- Misses        792      795       +3     
  Partials      121      121

Impacted Files	Coverage Δ
reflect_map.go	87.68% <100%> (+0.18%)	⬆️
reflect_struct_decoder.go	48.74% <0%> (-0.42%)	⬇️

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