[release-23.0] evalengine: Fix NULL document handling in JSON functions (#19052)

[vitess-bot]

Description

This is a backport of #19052

[vitess-bot]

Hello @arthurschreiber, there are conflicts in this backport.

Please address them in order to merge this Pull Request. You can execute the snippet below to reset your branch and resolve the conflict manually.

Make sure you replace origin by the name of the vitessio/vitess remote

git fetch --all
gh pr checkout 19231
git reset --hard origin/release-23.0
git cherry-pick -m 1 43ba94b3fb718c3b13db5a792a13c509c73d2b37

[arthurschreiber]

This depends on #19254 being merged first.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.72%. Comparing base (7282f6a) to head (bf3f021).
⚠️ Report is 2 commits behind head on release-23.0.

Files with missing lines	Patch %	Lines
go/vt/vtgate/evalengine/fn_json.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-23.0   #19231      +/-   ##
================================================
+ Coverage         69.70%   69.72%   +0.02%     
================================================
  Files              1606     1606              
  Lines            214792   214800       +8     
================================================
+ Hits             149720   149770      +50     
+ Misses            65072    65030      -42     

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