Fix possible nullref

when `ProviderManager.SaveMetadata` gets called before
`ProviderManager.AddParts`

```
Error Message:
   System.ArgumentNullException : Value cannot be null. (Parameter 'source')
  Stack Trace:
     at System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
   at System.Linq.Enumerable.Where[TSource](IEnumerable`1 source, Func`2 predicate)
   at MediaBrowser.Providers.Manager.ProviderManager.SaveMetadata(BaseItem item, ItemUpdateType updateType, IEnumerable`1 savers) in D:\a\1\s\MediaBrowser.Providers\Manager\ProviderManager.cs:line 674
   at MediaBrowser.Providers.Manager.ProviderManager.SaveMetadata(BaseItem item, ItemUpdateType updateType) in D:\a\1\s\MediaBrowser.Providers\Manager\ProviderManager.cs:line 655
   at Emby.Server.Implementations.Library.LibraryManager.RunMetadataSavers(BaseItem item, ItemUpdateType updateReason) in D:\a\1\s\Emby.Server.Implementations\Library\LibraryManager.cs:line 2012
   at Emby.Server.Implementations.Library.LibraryManager.UpdateItemsAsync(IReadOnlyList`1 items, BaseItem parent, ItemUpdateType updateReason, CancellationToken cancellationToken) in D:\a\1\s\Emby.Server.Implementations\Library\LibraryManager.cs:line 1970
   at Emby.Server.Implementations.Library.LibraryManager.CreateRootFolder() in D:\a\1\s\Emby.Server.Implementations\Library\LibraryManager.cs:line 775
   at Emby.Server.Implementations.Library.LibraryManager.get_RootFolder() in D:\a\1\s\Emby.Server.Implementations\Library\LibraryManager.cs:line 180
   at Emby.Server.Implementations.IO.LibraryMonitor.Start() in D:\a\1\s\Emby.Server.Implementations\IO\LibraryMonitor.cs:line 135
   at Emby.Server.Implementations.IO.LibraryMonitorStartup.RunAsync() in D:\a\1\s\Emby.Server.Implementations\IO\LibraryMonitorStartup.cs:line 26
   at Emby.Server.Implementations.ApplicationHost.StartEntryPoints(IEnumerable`1 entryPoints, Boolean isBeforeStartup)+MoveNext() in D:\a\1\s\Emby.Server.Implementations\ApplicationHost.cs:line 541
   at System.Threading.Tasks.Task.WhenAll(IEnumerable`1 tasks)
   at Emby.Server.Implementations.ApplicationHost.RunStartupTasksAsync(CancellationToken cancellationToken) in D:\a\1\s\Emby.Server.Implementations\ApplicationHost.cs:line 525
   at Jellyfin.Server.Integration.Tests.JellyfinApplicationFactory.CreateServer(IWebHostBuilder builder) in D:\a\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.OpenApiSpecTests.GetSpec_ReturnsCorrectResponse() in D:\a\1\s\tests\Jellyfin.Server.Integration.Tests\OpenApiSpecTests.cs:line 26
--- End of stack trace from previous location ---
```

MediaBrowser.Controller/Providers/IProviderManager.cs

@@ -91,8 +91,11 @@ namespace MediaBrowser.Controller.Providers
         /// <summary>
         /// Adds the metadata providers.
         /// </summary>
-        void AddParts(IEnumerable<IImageProvider> imageProviders, IEnumerable<IMetadataService> metadataServices, IEnumerable<IMetadataProvider> metadataProviders,
-            IEnumerable<IMetadataSaver> savers,
+        void AddParts(
+            IEnumerable<IImageProvider> imageProviders,
+            IEnumerable<IMetadataService> metadataServices,
+            IEnumerable<IMetadataProvider> metadataProviders,
+            IEnumerable<IMetadataSaver> metadataSavers,
             IEnumerable<IExternalId> externalIds);
 
         /// <summary>

MediaBrowser.Providers/Manager/ProviderManager.cs

@@ -60,8 +60,8 @@ namespace MediaBrowser.Providers.Manager
 
         private IMetadataService[] _metadataServices = Array.Empty<IMetadataService>();
         private IMetadataProvider[] _metadataProviders = Array.Empty<IMetadataProvider>();
-        private IEnumerable<IMetadataSaver> _savers;
-        private IExternalId[] _externalIds;
+        private IMetadataSaver[] _savers = Array.Empty<IMetadataSaver>();
+        private IExternalId[] _externalIds = Array.Empty<IExternalId>();
         private bool _isProcessingRefreshQueue;
         private bool _disposed;
 
@@ -125,7 +125,7 @@ namespace MediaBrowser.Providers.Manager
             _externalIds = externalIds.OrderBy(i => i.ProviderName).ToArray();
 
             _savers = metadataSavers
-                .Where(i => !(i is IConfigurableProvider configurable) || configurable.IsEnabled)
+                .Where(i => i is not IConfigurableProvider configurable || configurable.IsEnabled)
                 .ToArray();
         }