Improved testing of JSON IO, especially in de code that handles null objects"

[jurgenvinju]

• test specific null values for all types in the default null map
• deal with top-level null values that are not in the null map
• deal with nested null values that are not in the null map
  • as part of lists
  • as part of maps
  • as part of sets
  • as fields of a tuple
  • as positional fields of nodes
  • as positional fields of ADTs
  • as keyword field values
  • as part of source location unpacked elements
• fix bugs that pop up

[codecov]

Codecov Report

Attention: Patch coverage is 78.84615% with 11 lines in your changes missing coverage. Please review.

Project coverage is 49%. Comparing base (6c58baa) to head (7dbb8be).
Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
...pl/library/lang/json/internal/JsonValueReader.java	78%	6 Missing and 5 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #2116   +/-   ##
=======================================
  Coverage       49%     49%           
- Complexity    6224    6240   +16     
=======================================
  Files          663     663           
  Lines        59181   59211   +30     
  Branches      8615    8627   +12     
=======================================
+ Hits         29458   29531   +73     
+ Misses       27497   27454   -43     
  Partials      2226    2226           

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