fix: Allow cursor_string to be null in PlaylistScores

[Unknown15082]

Querying all scores in a room's playlist can return "cursor_string": null, which causes playlist_scores to panic since it expected a string. To fix this, I changed cursor to be optional.

Please tell me if this issue is not reproducible, or if there's anything I missed.

[MaxOhn]

Thanks for the PR!

I think specifying the cursor in the get_next method needs an adjustment since using the default string (i.e. an empty one) as cursor doesn't necessarily work with the api. Something like this should be better I think

let mut req = osu.playlist_scores(..).limit(..).sort(..);
            
if let Some(cursor) = self.cursor.as_deref() {
    req = req.cursor(cursor);
}

Some(req.await)