Cache mutation nullability bug

[AnthonyMDev]

Closes #2697

This PR:

• Adds a handler to the GraphQLAccumulator protocol for accepting values that are expected to exist but are not in the JSON object.
• The SelectionSetMapper accumulator now has options for maintaining null values instead of stripping them out and allowing missing values to be accepted for optional fields.
  • These are used when reading cache values onto a mutable selection set for cache updates, to maintain existing null values and allowing updates to cache objects that are currently incomplete.

Future work to allow fields from cache mutations to be cleared out is enabled by this PR as well.

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[calvincestari]

Nice work, thanks!