`cursors` in `rspotify_model::page::CursorBasedPage` can be `None`

[aome510]

Describe the bug The cursors field in the cursor-based page object can be null if the page returned by the request is empty. Assuming it is non-nullable makes deserializing the response object panic.

To Reproduce Steps to reproduce the behavior:

1. Make a get-recently-played request

   curl --request GET \
    --url 'https://api.spotify.com/v1/me/player/recently-played?before=<random number, e.g 100>' \
    --header 'Authorization: Bearer <TOKEN> \
    --header 'Content-Type: application/json'

Expected behavior Found the cursors field in the response to be null.

[marioortizmanero]

Can confirm that I can reproduce this:

 curl --request GET \
     --url 'https://api.spotify.com/v1/me/player/recently-played?before=100' \
     --header "Authorization: Bearer $ACCESS_TOKEN" \
     --header 'Content-Type: application/json'
{
  "items" : [ ],
  "next" : null,
  "cursors" : null,
  "limit" : 20,
  "href" : "https://api.spotify.com/v1/me/player/recently-played?before=100"
}