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

MediaBrowser.Controller/Entities/AggregateFolder.cs

@@ -187,14 +187,14 @@ namespace MediaBrowser.Controller.Entities
         /// <exception cref="ArgumentNullException">The id is empty.</exception>
         public BaseItem FindVirtualChild(Guid id)
         {
-            if (id.Equals(Guid.Empty))
+            if (id.Equals(default))
             {
                 throw new ArgumentNullException(nameof(id));
             }
 
             foreach (var child in _virtualChildren)
             {
-                if (child.Id == id)
+                if (child.Id.Equals(id))
                 {
                     return child;
                 }