PR for v.1.7.4

[hcpchris]

• BUGFIX: Fixed incorrect .to_json() serialization of null values inside of an iterable.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.30%. Comparing base (f8ee1ba) to head (6072807).
Report is 1 commits behind head on master.

Files	Patch %	Lines
highcharts_core/metaclasses.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
- Coverage   88.31%   88.30%   -0.01%     
==========================================
  Files         209      209              
  Lines       30899    30901       +2     
  Branches     2498     2499       +1     
==========================================
  Hits        27287    27287              
- Misses       2673     2674       +1     
- Partials      939      940       +1     

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