Update checkNullElementsInArray() unit test to check both error message and error code instead of only checking the error code

[afazel]

opensearch-security pull request intake form

Please provide as much details as possible to get feedback/acceptance on your PR quickly

1. Category: (Enhancement, New feature, Bug fix, Test fix, Refactoring, Maintenance, Documentation)
   Bug fix

2. Github Issue # or road-map entry, if available:

3. Description of changes:
   Currently checkNullElementsInArray() unit test only checks for the error code and it will pass even if request fails for a different reason. We need to check the error message to make sure that the message is also what we expect.

4. Why these changes are required?
   To make sure that the test is not passing if the request fails for a different reason.

5. What is the old behavior before changes and new behavior after changes? (Please add any example/logs/screen-shot if available)
   In the new behavior, the unit test will pass only if error code and the error message are what we expected.

6. Testing done: (Please provide details of testing done: Unit testing, integration testing and manual testing)

7. TO-DOs, if any: (Please describe pending items and provide Github issues# for each of them)

8. Is it backport from main branch? (If yes, please add backport PR # and commits #)

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[codecov-commenter]

Codecov Report

Merging #1370 (c37e18c) into main (9f04635) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1370   +/-   ##
=========================================
  Coverage     64.80%   64.81%           
- Complexity     3201     3202    +1     
=========================================
  Files           247      247           
  Lines         17243    17243           
  Branches       3050     3050           
=========================================
+ Hits          11175    11176    +1     
  Misses         4517     4517           
+ Partials       1551     1550    -1     

Impacted Files	Coverage Δ
...a/org/opensearch/security/tools/SecurityAdmin.java	47.52% <0.00%> (+0.13%)	⬆️

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