Port 'missing field' error fix to C#

[nharper285]

Summary of the Pull Request

What is this about?
A 'missing field' error affecting libfuzzer was previously addressed in #1739. This fix addressed #1672.

This PR ports the fix to the C# code.
Closes #2883

PR Checklist

• Applies to work item: Onefuzz reports a 'missing field' error for the optional field preserve_existing_outputs #1672
• 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

What does this include?

Validation Steps Performed

How does someone test & validate?

[codecov-commenter]

Codecov Report

Merging #2905 (dd28163) into main (091c870) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2905      +/-   ##
==========================================
- Coverage   28.22%   28.21%   -0.01%     
==========================================
  Files         303      303              
  Lines       36013    36018       +5     
==========================================
  Hits        10164    10164              
- Misses      25849    25854       +5     

Impacted Files	Coverage Δ
src/ApiService/ApiService/OneFuzzTypes/Model.cs	69.35% <0.00%> (-0.09%)	⬇️
src/ApiService/ApiService/onefuzzlib/Config.cs	0.00% <0.00%> (ø)
src/ApiService/ApiService/onefuzzlib/Defs.cs	0.00% <0.00%> (ø)

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