Handle nulls better in scaleset shutdown

[tevoinea]

Summary of the Pull Request

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PR Checklist

• Applies to work item: #xxx
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

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Validation Steps Performed

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[codecov-commenter]

Codecov Report

Merging #2538 (635afe8) into main (f313a37) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2538      +/-   ##
==========================================
- Coverage   29.84%   29.84%   -0.01%     
==========================================
  Files         288      288              
  Lines       35677    35680       +3     
==========================================
  Hits        10648    10648              
- Misses      25029    25032       +3     

Impacted Files	Coverage Δ
...ervice/ApiService/onefuzzlib/ScalesetOperations.cs	2.66% <0.00%> (-0.01%)	⬇️
...ApiService/ApiService/onefuzzlib/VmssOperations.cs	0.00% <0.00%> (ø)

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