[release/5.0] Fix NRE introduced in FileSystemWatcher during nullability annotations

[github-actions]

Backport of #41315 to release/5.0

/cc @danmosemsft @krwq

Customer Impact

Testing

Risk

[ghost]

Hello @danmosemsft!

Because this pull request has the auto-merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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[danmoseley]

@krwq please own this one -- you can merge when green.

[ghost]

Apologies, while this PR appears ready to be merged, I've been configured to only merge when all checks have explicitly passed. The following integrations have not reported any progress on their checks and are blocking auto-merge:

1. DotNet Maestro - Int
2. DotNet Maestro
3. Dependabot

These integrations are possibly never going to report a check, and unblocking auto-merge likely requires a human being to update my configuration to exempt these integrations from requiring a passing check.

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