[WIP] Instruct TS that reading from cache could sometime give null

[kamilkisiela]

Fixes #2503

[apollo-cla]

	Warnings
⚠️	There are library changes, but not tests. That's OK as long as you're refactoring existing code

	Messages
📖	Please add your name and email to the AUTHORS file (optional)

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[kamilkisiela]

@jbaxleyiii I'm pretty sure it won't break any library or any app that uses apollo-client or apollo-cache but I haven't checked it. That's why I marked it as WIP and in-progress but feel free to merge it.

[jbaxleyiii]

@nevir @yuit will this break hermes (I'd assume so?)

[nevir]

Probably! But a worthy change 👍 No prob