Fix: Initialize _computerName to "."

[seer-by-sentry]

Resolved / Related Issues

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• Closes FILES-APP-454

Steps used to test these changes

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1. The issue was identified in Sentry (https://sentry.io/organizations/files-org/issues/6775648189/) where CimSession.Create receives a null computer name because the ManagementEventWatcher constructor used does not initialize the computer name field.
2. The fix initializes the _computerName field to "." in the constructors of ManagementEventWatcher.
3. This ensures that the local computer is targeted by default when no computer name is explicitly provided.