Handle NoneType in elastic adapter document_stream

[JSCU-CNI]

This PR fixes a Unpackable type <class 'NoneType'> exception for the elastic adapter. This can occur when the record queue gets populated with None instead of a record - which apparently can occur if for example a Dissect plugin does not return any records.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.85%. Comparing base (b91f216) to head (a3705a9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
flow/record/adapter/elastic.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
- Coverage   83.90%   83.85%   -0.05%     
==========================================
  Files          34       34              
  Lines        3485     3487       +2     
==========================================
  Hits         2924     2924              
- Misses        561      563       +2     

Flag	Coverage Δ
unittests	83.85% <0.00%> (-0.05%)	⬇️

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[yunzheng]

Do you know which dissect plugin by any chance? It sounds like this should also be fixed there.

[JSCU-CNI]

We'll look into that in the dissect.target project.