`relayStylePagination` `read` method clobbers `pageInfo.startCursor` and `pageInfo.endCursor`

Intended outcome: With a query as follows:

query {
  someConnectionField {
    edges {
      node {
        id
      }
    }
    pageInfo {
      hasNextPage
      endCursor
    }
  }
}

I would expect endCursor to be the value returned from the API, not "".

Actual outcome: Even though the API returns a valid cursor in endCursor, the relayStylePagination type policy attempts to read it from edges[0].cursor (or the last edge’s cursor) and completely replaces the one in `pageInfo.

These should default to pageInfo.{startCursor,endCursor} and not "". https://github.com/apollographql/apollo-client/blob/cefb3c32621de6973191c7abb6edd365d53cbf41/src/utilities/policies/pagination.ts#L101-L102

How to reproduce the issue: Perform a query as described above while using relayStylePagination.

[dlqqq]

@benjamn Gave your PR a quick glance, it looks good to me. Glad it was (relatively) easy! I was just about to submit a PR earlier, but you beat me to it 😅 . TYSM!

[benjamn]

@migueloller @diracs-delta @meliborn Please have a look at this PR and let me know if it meets your expectations: #8438