Update dotnet-isolated-process-guide.md

[felixtsem5]

This is annoying me enough times that I'm going to do a PR.
This will get rid of the warning CS8600: Converting null literal or possible null value to non-nullable type.
The FirstOrDefault method returns nullable LoggerFilterRule? type.

[prmerger-automator]

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[ttorble]

@ggailey777

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
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