fix: Null expr does not work for json field

[sunby]

issue: #40455

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 70.03%. Comparing base (a56b240) to head (aac5c12).

Files with missing lines	Patch %	Lines
internal/core/src/mmap/ChunkedColumn.h	50.00%	3 Missing ⚠️
...rnal/core/src/segcore/ChunkedSegmentSealedImpl.cpp	0.00%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (40.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (70.03%) is below the target coverage (77.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #40456      +/-   ##
==========================================
- Coverage   70.04%   70.03%   -0.02%     
==========================================
  Files         309      309              
  Lines       27725    27725              
==========================================
- Hits        19421    19417       -4     
- Misses       8304     8308       +4     

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	70.03% <40.00%> (-0.02%)	⬇️
Go	∅ <ø> (∅)

Files with missing lines	Coverage Δ
internal/core/src/storage/ChunkCache.cpp	78.36% <100.00%> (-2.34%)	⬇️
internal/core/src/mmap/ChunkedColumn.h	51.51% <50.00%> (ø)
...rnal/core/src/segcore/ChunkedSegmentSealedImpl.cpp	24.31% <0.00%> (ø)

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