Fix artist metadata not being fetched on initial library scan (#16606)

* Fix artist metadata not being fetched on initial library scan

* Update Emby.Server.Implementations/Library/Validators/ArtistsValidator.cs

Co-authored-by: Bond-009 <bond.009@outlook.com>

---------

Co-authored-by: Bond-009 <bond.009@outlook.com>

MediaBrowser.Providers/Manager/ProviderManager.cs

@@ -487,6 +487,13 @@ namespace MediaBrowser.Providers.Manager
                 return true;
             }
 
+            // Artists without a folder structure that are derived from metadata have no real path in the library,
+            // so GetLibraryOptions returns null. Allow all providers through rather than blocking them.
+            if (item is MusicArtist && libraryTypeOptions is null)
+            {
+                return true;
+            }
+
             return _baseItemManager.IsMetadataFetcherEnabled(item, libraryTypeOptions, provider.Name);
         }