Export of content type on save fails when created in code and AllowedContentTypes is set to null

[Luuk1983]

Describe the bug
When creating a content type in code and setting the AllowedContentTypes property of the ContentType object to null, uSync will throw an error when saving this content type.

System.ArgumentNullException: Value cannot be null. (Parameter 'source')
   at System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
   at System.Linq.OrderedEnumerable`2..ctor(IEnumerable`1 source, Func`2 keySelector, IComparer`1 comparer, Boolean descending, OrderedEnumerable`1 parent)
   at System.Linq.Enumerable.OrderBy[TSource,TKey](IEnumerable`1 source, Func`2 keySelector)
   at uSync.Core.Serialization.Serializers.ContentTypeBaseSerializer`1.SerializeStructure(TObject item)
   at uSync.Core.Serialization.Serializers.ContentTypeSerializer.SerializeCore(IContentType item, SyncSerializerOptions options)
   at uSync.Core.Serialization.SyncSerializerRoot`1.Serialize(TObject item, SyncSerializerOptions options)
   at uSync.BackOffice.SyncHandlers.SyncHandlerRoot`2.SerializeItem(TObject item, SyncSerializerOptions options)
   at uSync.BackOffice.SyncHandlers.SyncHandlerRoot`2.Export_DoExport(TObject item, String filename, String[] folders, HandlerSettings config)
   at uSync.BackOffice.SyncHandlers.SyncHandlerRoot`2.Export(TObject item, String[] folders, HandlerSettings config)
   at uSync.BackOffice.SyncHandlers.SyncHandlerRoot`2.Handle(SavedNotification`1 notification)

To Reproduce
Steps to reproduce the behavior:
Create these two document types in code. It doesn't matter where; in a razor view, in a controller or in a composition:

var result = new ContentType(stringHelper, -1)
{
	Alias = "WorkingComposition",
	Description = "This composition works",
	Icon = "icon-home",
	Name = "Working composition",
};

contentTypeService.Save(result);

var result2 = new ContentType(stringHelper, -1)
{
	Alias = "NonWorkingComposition",
	Description = "This composition doesn't work",
	Icon = "icon-home color-red",
	Name = "Not working composition",
	AllowedContentTypes = null
};

contentTypeService.Save(result2);

When running this code, uSync will create the working composition file, but not the one where the AlloweedContentTypes are set to null:

Expected behavior
Because AllowedContentTypes is nullable I full expect it to be able to be set to null.

About your Site (please complete the following information):

• Umbraco Version: 13.3.2
• uSync Version: 13.2.3

[KevinJump]

Fixed in uSync 13.2.4 : https://github.com/KevinJump/uSync/releases