fix: support more types of nullable elements in response/entity cache keys

[carodewig]

#8767 changed the entity and response caching keys to support nullable elements. The implementation covered the case of a field explicitly being set to null, but did not cover the following cases:

• Nullable field being missing
• Nullable list items

This PR adds support for those cases and corresponding tests.

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[apollo-librarian]

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* graphos/routing/(latest)/operations/subscriptions/api-gateway.mdx
* graphos/routing/(latest)/security/router-authentication.mdx

Build ID: 3dcc66355a9a55120b013396
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URL: https://www.apollographql.com/docs/deploy-preview/3dcc66355a9a55120b013396

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