Optimize Guid comparisons

* Use Guid.Equals(Guid) instead of the == override
* Ban the usage of Guid.Equals(Object) to prevent accidental boxing
* Compare to default(Guid) instead of Guid.Empty

Jellyfin.Api/Controllers/PlaylistsController.cs

@@ -181,7 +181,9 @@ namespace Jellyfin.Api.Controllers
                 return NotFound();
             }
 
-            var user = !userId.Equals(Guid.Empty) ? _userManager.GetUserById(userId) : null;
+            var user = userId.Equals(default)
+                ? null
+                : _userManager.GetUserById(userId);
 
             var items = playlist.GetManageableItems().ToArray();