Fix possible ArgumentNullException

```
  Error Message:
   System.ArgumentNullException : Value cannot be null. (Parameter 'source')
  Stack Trace:
     at System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
   at System.Linq.Enumerable.Select[TSource,TResult](IEnumerable`1 source, Func`2 selector)
   at Emby.Server.Implementations.Library.LibraryManager.ResolveItem(ItemResolveArgs args, IItemResolver[] resolvers) in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 475
   at Emby.Server.Implementations.Library.LibraryManager.ResolvePath(FileSystemMetadata fileInfo, IDirectoryService directoryService, IItemResolver[] resolvers, Folder parent, String collectionType, LibraryOptions libraryOptions) in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 618
   at Emby.Server.Implementations.Library.LibraryManager.ResolvePath(FileSystemMetadata fileInfo, Folder parent) in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 536
   at Emby.Server.Implementations.Library.LibraryManager.GetUserRootFolder() in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 812
   at Emby.Server.Implementations.Library.LibraryManager.GetCollectionFolders(BaseItem item) in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 2080
   at Emby.Server.Implementations.Library.LibraryManager.GetLibraryOptions(BaseItem item) in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 2116
   at MediaBrowser.Providers.Manager.ProviderManager.SaveMetadata(BaseItem item, ItemUpdateType updateType, IEnumerable`1 savers) in /home/vsts/work/1/s/MediaBrowser.Providers/Manager/ProviderManager.cs:line 672
   at MediaBrowser.Providers.Manager.ProviderManager.SaveMetadata(BaseItem item, ItemUpdateType updateType) in /home/vsts/work/1/s/MediaBrowser.Providers/Manager/ProviderManager.cs:line 655
   at Emby.Server.Implementations.Library.LibraryManager.RunMetadataSavers(BaseItem item, ItemUpdateType updateReason) in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 2017
   at Emby.Server.Implementations.Library.LibraryManager.UpdateItemsAsync(IReadOnlyList`1 items, BaseItem parent, ItemUpdateType updateReason, CancellationToken cancellationToken) in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 1975
   at Emby.Server.Implementations.Library.LibraryManager.CreateRootFolder() in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 775
   at Emby.Server.Implementations.Library.LibraryManager.get_RootFolder() in /home/vsts/work/1/s/Emby.Server.Implementations/Library/LibraryManager.cs:line 180
   at Emby.Server.Implementations.IO.LibraryMonitor.Start() in /home/vsts/work/1/s/Emby.Server.Implementations/IO/LibraryMonitor.cs:line 135
   at Emby.Server.Implementations.IO.LibraryMonitorStartup.RunAsync() in /home/vsts/work/1/s/Emby.Server.Implementations/IO/LibraryMonitorStartup.cs:line 26
   at Emby.Server.Implementations.ApplicationHost.StartEntryPoints(IEnumerable`1 entryPoints, Boolean isBeforeStartup)+MoveNext() in /home/vsts/work/1/s/Emby.Server.Implementations/ApplicationHost.cs:line 518
   at System.Threading.Tasks.Task.WhenAll(IEnumerable`1 tasks)
   at Emby.Server.Implementations.ApplicationHost.RunStartupTasksAsync(CancellationToken cancellationToken) in /home/vsts/work/1/s/Emby.Server.Implementations/ApplicationHost.cs:line 502
   at Jellyfin.Server.Integration.Tests.JellyfinApplicationFactory.CreateServer(IWebHostBuilder builder) in /home/vsts/work/1/s/tests/Jellyfin.Server.Integration.Tests/JellyfinApplicationFactory.cs:line 101
   at Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory`1.EnsureServer()
   at Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory`1.CreateDefaultClient(DelegatingHandler[] handlers)
   at Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory`1.CreateDefaultClient(Uri baseAddress, DelegatingHandler[] handlers)
   at Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory`1.CreateClient(WebApplicationFactoryClientOptions options)
   at Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory`1.CreateClient()
   at Jellyfin.Server.Integration.Tests.Controllers.ActivityLogControllerTests.ActivityLog_GetEntries_Ok() in /home/vsts/work/1/s/tests/Jellyfin.Server.Integration.Tests/Controllers/ActivityLogControllerTests.cs:line 21
--- End of stack trace from previous location ---
```

Emby.Server.Implementations/Library/LibraryManager.cs

@@ -196,13 +196,13 @@ namespace Emby.Server.Implementations.Library
         /// Gets or sets the postscan tasks.
         /// </summary>
         /// <value>The postscan tasks.</value>
-        private ILibraryPostScanTask[] PostscanTasks { get; set; }
+        private ILibraryPostScanTask[] PostscanTasks { get; set; } = Array.Empty<ILibraryPostScanTask>();
 
         /// <summary>
         /// Gets or sets the intro providers.
         /// </summary>
         /// <value>The intro providers.</value>
-        private IIntroProvider[] IntroProviders { get; set; }
+        private IIntroProvider[] IntroProviders { get; set; } = Array.Empty<IIntroProvider>();
 
         /// <summary>
         /// Gets or sets the list of entity resolution ignore rules.
@@ -214,15 +214,15 @@ namespace Emby.Server.Implementations.Library
         /// Gets or sets the list of currently registered entity resolvers.
         /// </summary>
         /// <value>The entity resolvers enumerable.</value>
-        private IItemResolver[] EntityResolvers { get; set; }
+        private IItemResolver[] EntityResolvers { get; set; } = Array.Empty<IItemResolver>();
 
-        private IMultiItemResolver[] MultiItemResolvers { get; set; }
+        private IMultiItemResolver[] MultiItemResolvers { get; set; } = Array.Empty<IMultiItemResolver>();
 
         /// <summary>
         /// Gets or sets the comparers.
         /// </summary>
         /// <value>The comparers.</value>
-        private IBaseItemComparer[] Comparers { get; set; }
+        private IBaseItemComparer[] Comparers { get; set; } = Array.Empty<IBaseItemComparer>();
 
         public bool IsScanRunning { get; private set; }