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

Emby.Server.Implementations/EntryPoints/LibraryChangedNotifier.cs

@@ -326,7 +326,7 @@ namespace Emby.Server.Implementations.EntryPoints
         {
             var userIds = _sessionManager.Sessions
                 .Select(i => i.UserId)
-                .Where(i => !i.Equals(Guid.Empty))
+                .Where(i => !i.Equals(default))
                 .Distinct()
                 .ToArray();