Fix issue with scheduling embedded documents for upsert in the UnitOfWork. It results in mongodb creating a {} for an @EmbedMany which results in a "Cannot apply $push/$pushAll modifier to non-array" error when we try to $push to it later.
EmbedMany with mapped identifier and strategy="none" causes notice "Undefined index: id" in DocumentPersister.php line 593
null reference $id should throw DocumentNotFoundException
Fix EmbedOne with mapped identifier and strategy="none" causes notice.
Test registerManaged() with embedded documents and null ids when mapped and not mapped.