Fix crash when running JsonSerializer::closeNested() while in write mode

[AndreasLrx]

Description

FIx a crash occuring when calling JsonSerializer::closeNested() while writing data to the serializer due to a missing verification.

Also introduce new isArrayEnd and getArraySize helper methods.

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Added/updated tests?

• Didn't had the time to write them, and I am the maintainer
• New tests written
• Existing tests updated
• Tests are not required because this is a documentation update
• I need help with writing tests

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.36%. Comparing base (4f4cb2a) to head (f093c24).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #187   +/-   ##
=======================================
  Coverage   82.36%   82.36%           
=======================================
  Files          70       70           
  Lines        1242     1242           
  Branches      383      383           
=======================================
  Hits         1023     1023           
  Misses        148      148           
  Partials       71       71           

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