Fix: Prevent null reference exceptions in path update logic

[seer-by-sentry]

Resolved / Related Issues

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• Closes FILES-APP-4GW

Steps used to test these changes

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1. The issue was that: Null path passed to UI update causes ObservableCollection modification, leading to NullReferenceException during WinRT event marshaling.
2. Added validation to prevent null reference exceptions when updating the path UI with a new working directory or library name.
3. Ensures that newWorkingDir and e.Name are not null or whitespace before being used in UpdatePathUIToWorkingDirectoryAsync.

This fix was generated by Seer in Sentry, triggered by Yair. 👁️ Run ID: 3595863

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