Handle the audit config being null

[merlinz01]

This was causing an error if the audit config was null and was posted to the event bus.

Description

• Category: Bugfix
• Why these changes are required? An empty audit config causes the logs to be filled with an irrelevant error.
• What is the old behavior before changes and new behavior after changes?
  Old behavior:
  Lots of errors when the audit config is missing
  New behavior:
  Audit config is set to the default when missing

Issues Resolved

#4315
https://forum.opensearch.org/t/ava-lang-nullpointerexception-cannot-invoke-object-getclass-because-event-is-null/20821/

Is this a backport? No.
Do these changes introduce new permission(s) to be displayed in the static dropdown on the front-end? No.

Testing

None

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[cwperks]

reason: AuditConfig() has private access in AuditConfig

Would a different constructor work? i.e. new AuditConfig(false, Filter.DEFAULT, ComplianceConfig.DEFAULT)?

[merlinz01]

Trying AuditConfig.from(Settings.EMPTY) as done here for ComplianceConfig.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.26%. Comparing base (ec9fadb) to head (438afdf).
Report is 7 commits behind head on main.

Files	Patch %	Lines
...ch/security/securityconf/DynamicConfigFactory.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4650   +/-   ##
=======================================
  Coverage   65.26%   65.26%           
=======================================
  Files         317      317           
  Lines       22311    22312    +1     
  Branches     3588     3589    +1     
=======================================
+ Hits        14562    14563    +1     
  Misses       5954     5954           
  Partials     1795     1795           

Files	Coverage Δ
...ch/security/securityconf/DynamicConfigFactory.java	55.29% <50.00%> (-0.33%)	⬇️

... and 2 files with indirect coverage changes