Fix handling of GlobalOptions deserializing to null

[rkm]

Proposed Changes

Provides a much better message in some cases e.g., specifying an empty file.

This highlighted that the Nullable feature doesn't save us unless we specify that the template type is also nullable.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-Only Update (if none of the other choices apply)
  • In this case, ensure that the message of the head commit from the source branch is prefixed with [skip ci]

Checklist

By opening this PR, I confirm that I have:

• Reviewed the contributing guidelines for this repository
• Ensured that the PR branch is in sync with the target branch (i.e. it is automatically merge-able)
• Updated any relevant API documentation
• Created or updated any tests if relevant
• Created a news file
  • NOTE: This must include any changes to any of the following files: default.yaml, any of the RabbitMQ server configurations, GlobalOptions.cs
• Listed myself in the CONTRIBUTORS file 🚀
• Requested a review by the @SMI/Reviewers team

Issues

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.07%. Comparing base (907f10e) to head (4deaf6c).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1917      +/-   ##
==========================================
+ Coverage   64.02%   64.07%   +0.05%     
==========================================
  Files         177      177              
  Lines        6527     6525       -2     
  Branches      967      968       +1     
==========================================
+ Hits         4179     4181       +2     
+ Misses       2059     2057       -2     
+ Partials      289      287       -2     

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