Use JsonNull for null values to ensure rendering in sfn state machine definition

[rpmcginty]

What's in this Change?

It turns out that when aws_cdk.aws_stepfunctions code renders state machine definitions, it goes through all the code definitions and removes any null or undefined (which is None in our case). This means that despite us specifically specifying a field in a a parameter of a pass state, this value gets stripped out.

The solution is to fill in this value with a JsonNull.INSTANCE. It preserves the null value when rendered as cloud formation.

Testing

I synthesized results and confirmed the existence of the null value once I made this change:

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.54%. Comparing base (c9668d2) to head (fba413b).

Files with missing lines	Patch %	Lines
...rmatics_cdk_lib/constructs_/sfn/fragments/batch.py	33.33%	2 Missing ⚠️
...lib/constructs_/sfn/fragments/informatics/batch.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   52.51%   52.54%   +0.03%     
==========================================
  Files          58       58              
  Lines        2866     2868       +2     
  Branches      246      246              
==========================================
+ Hits         1505     1507       +2     
  Misses       1334     1334              
  Partials       27       27              

Files with missing lines	Coverage Δ
...lib/constructs_/sfn/fragments/informatics/batch.py	41.37% <50.00%> (+0.68%)	⬆️
...rmatics_cdk_lib/constructs_/sfn/fragments/batch.py	27.02% <33.33%> (+0.99%)	⬆️

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