Handle cases gracefully where null field name is passed to LeafReader methods

[nibix]

Description

In a couple of cases, OpenSearch calls LeafReader methods with null fieldName parameters. The semantics of this are a bit unclear; in most cases, the underlying LeafReader methods will just also return null. However, I cannot prove that this will be always the case.

Thus, the change in this PR does not try to put semantics into this; it just handles the null case gracefully by using a NPE safe replacement value. That has the effect that users with high restrictions will also get an restriction enforced on the result returned in the null cases. Users without restrictions will be still passed through to the underlying LeafReader.

See also #5504

• Category: Bug fix
• Why these changes are required? Diverse NPE issues
• What is the old behavior before changes and new behavior after changes?

Issues Resolved

Fixes opensearch-project/OpenSearch-Dashboards#9873

Testing

• Unit test
• Manual test

Check List

• New functionality includes testing
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.95%. Comparing base (25fe5f8) to head (9ef236e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5537      +/-   ##
==========================================
+ Coverage   72.88%   72.95%   +0.06%     
==========================================
  Files         400      405       +5     
  Lines       24935    25161     +226     
  Branches     3806     3829      +23     
==========================================
+ Hits        18173    18355     +182     
- Misses       4912     4946      +34     
- Partials     1850     1860      +10     

Files with missing lines	Coverage Δ
...earch/security/privileges/dlsfls/FieldMasking.java	86.82% <100.00%> (-0.24%)	⬇️
...ch/security/privileges/dlsfls/FieldPrivileges.java	95.36% <100.00%> (+0.06%)	⬆️

... and 7 files with indirect coverage changes

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[cwperks]

Thank you for the quick fix @nibix !