feat(graphcache): Allow cache.resolve to return undefined for uncached fields

[kitten]

Summary

This updates cache.resolve to return undefined for uncached fields.

This has two reasons:

• Resolvers allow users to return undefined to signal a cache miss, however, this wasn't clear — both because of the documentation, and because cache.resolve itself never returns undefined. Hence, while it's popular to end a resolver in a cache.resolve call, this meant that a cache miss could never be caused on this field.
• Not being able to return undefined meant that the Cache interface virtually had little to no ways of actually letting a user check whether a field was cached or not. By returning undefined this now becomes clear

This isn't considered a breaking change since it shouldn't alter prior behaviour significantly:

• A user may now correctly see a cache miss, where before they accidentally didn't cause one
  • However, manually returning null still retains the prior behaviour obviously
• All methods now accept undefined, so the chainability is retained. When a method that was chainable receives undefined as its entity input, it will also typically return undefined, unless it's a write method.

Set of changes

• Allow cache.resolve to return undefined
• Update cache.link to better handle variable number of arguments
• Allow Entity argument type in Cache to accept undefined
• Update ensureLink to handle undefined values gracefully
• Update docs to clarify that resolvers are allowed to return undefined