First user lost

[EvilSumrak2049]

Can you please explain why last_uid is set as None in the FullSortEvalDataLoader class? After all, this leads to the first user simply not getting anywhere, neither to the self.uid2items_num array nor to the self.uid2history_item array, because the condition is written in _set_user_property: if uid is None:
return. I can't understand why it's done this way. Thanks in advance for any answer.