Make startCursor and endCursor nullable

This is a valid case when you have 0 results (that can mean there's a next/prev page, even though you asked for 0 records).

For the more info on the subject:

https://relay.dev/graphql/connections.htm#sec-undefined.PageInfo.Fields

Which mentions:

The fields startCursor and endCursor can be null if there are
no results.