allow querying connection metadata and then subsequent data in relayStylePagination

[anark]

Currently if you try to query relay connection metadata with no arguments

query todos{
  totalCount
}

And then query for the edges/data

query todos{
  totalCount
  edges {
    node {
      title
    }
  }
}

The cache will never have any todo items and both data and error returned by the cache will always be undefined.

This attempts to fix this.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@anark: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[benjamn]

@anark Thanks for jumping into this code, and for adding tests! I agree with most of these changes, but I have one request for further consideration (below).

[anark]

@benjamn Thanks for the feedback.

I've adjusted the implementation, extended the test coverage to cover this, and updated from main. Please let me know what else this needs.

[Ilaiwi]

I am dealing with an error that this PR could resolve here.

pagination.js:66 Uncaught TypeError: Cannot read property 'slice' of undefined
    at merge (pagination.js:66)
    at Policies.applyMerges (policies.js:294)
    at policies.js:316

I am getting this when I do a subscribe to more on a field with relay style pagination policy and attempting to update the cache where existing is passed as undefined to the merge function in the policy

[benjamn]

LGTM—thanks @anark!