Allow keyFields and keyArgs functions to return false

[CarsonF]

Previously the function was not allowed to return false even though false is supported

[apollo-cla]

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[CarsonF]

Dang I thought this could be a quick fix. All I know is

keyArgs: () => []

results in "Foo:{}"

But

keyArgs: () => false

results in "Foo".
This works fine and is needed in my code because I need "Foo" and "Foo:{}" to be consolidated into a single stored field.

[benjamn]

@CarsonF Your use case makes sense, and you're right that we already handle falsy returns from both keyFields and keyArgs functions, so this additional return type is safe. Thanks for catching this!

[benjamn]

On second thought, if you could look into those test failures, that'd be great. I'm also happy to jump in if you run out of time.

[CarsonF]

Thanks @benjamn! It would be lovely if you could jump in. I didn't study the implementation and am only ignoring the type errors in my codebase.
I'm guessing that string | undefined is actually ok as string | false | undefined since we have the falsey checks instead of nullish checks, right?

[CarsonF]

Beautiful!

[benjamn]

Thanks again for getting this started @CarsonF!