Our `GetFileSystemEntries` method will throw when enumerating the file system, but its callers might consider the unhandled exceptions as the whole path is not available. This would cause a single problematic file to fail the enumeration, and could lead to unexpected side effects.
HandleIOException gracefully by marking the files throwing as not exist to let the caller skip that file.
* Enable nullable
* Add a fast return to ReportFileSystemChanged when path should be ignored
* Use Span overloads of Path.* functions where possible
* IFileSystem: remove NormalizePath as Path.TrimEndingDirectorySeparator already checks if it's a root path
```
[15:01:24] [ERR] [55] Jellyfin.Server.Middleware.ExceptionMiddleware: Error processing request. URL GET /Environment/Drives.
System.UnauthorizedAccessException: Access to the path is denied.
---> System.IO.IOException: Operation not permitted
--- End of inner exception stack trace ---
at System.IO.DriveInfo.CheckStatfsResultAndThrowIfNecessary(Int32 result)
at System.IO.DriveInfo.get_TotalSize()
at Emby.Server.Implementations.IO.ManagedFileSystem.<>c.<GetDrives>b__32_0(DriveInfo d) in /home/bond/dev/jellyfin/Emby.Server.Implementations/IO/ManagedFileSystem.cs:line 583
at System.Linq.Enumerable.WhereSelectArrayIterator`2.ToList()
at Emby.Server.Implementations.IO.ManagedFileSystem.GetDrives() in /home/bond/dev/jellyfin/Emby.Server.Implementations/IO/ManagedFileSystem.cs:line 583
at Jellyfin.Api.Controllers.EnvironmentController.GetDrives() in /home/bond/dev/jellyfin/Jellyfin.Api/Controllers/EnvironmentController.cs:line 153
at lambda_method559(Closure , Object , Object[] )
at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()
```
