fix: `end_date` is always overwritten by opened file.

https://github.com/Leibniz-HBI/DBoeS-stats/blame/b7b9c52be9d3f4eb8dc6e48787936f8042260c6a/factli/get_leaderboard.py#L57

User expect to set an `end_date` via a argument but this always overwrittes it, even when the specified file is empty. 