Handle nullable connections in relayStylePagination

[lukel97]

This handles my edge case in #7822 (comment) where a nullable connection with a field policy set to relayStylePagination would throw an error whenever a null existing/incoming node was handled. The fix is pretty bone-headed, so if there's a better way to go about fixing this please let me know!

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]

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[lukel97]

Bump

[benjamn]

@bubba I took the liberty of rebasing this branch against main and making a few changes in #8733, preserving your authorship. Closing in favor of that PR (though maybe consider enabling pushes from maintainers when you open future PRs, if you want us to be able to push commits to your branch). Thanks!