Path provider windows crash fix

[stuartmorgan]

Failures to get a known folder that don't throw Windows exceptions need
to return null.

Fixes flutter/flutter#80712

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[stuartmorgan]

Unfortunately I couldn't find a reasonable way to unit test this until we have Dart-based registration; I was going to make the function overridable with a test version that would return an error code, but if I do that I would need to put the override in the class's API, which means that the stub would need a copy (to avoid analyzer failures), but the function signature has FFI types in it so can't exist in the stub.

[cbracken]

lgtm. I can't think of a nice way to make this testable without adding ugly public API. :(