Change the value type of Record from T to T | undefined

[Eyal-Shalev]

Currently Record<K,V> is defined as follows:

type Record<K extends keyof any, T> = {
    [P in K]: T;
};

I suggest changing it to this:

type Record<K extends keyof any, T> = {
    [P in K]: T | undefined;
};

This allows TypeScript to protects us better from unchecked indexed access.

I could use the noUncheckedIndexedAccess but it doesn't play nice with the @typescript-eslint/no-unnecessary-condition eslint rule, as according to the type checker myRecord[someKey] will always return a T.