Re: [xhr] FormData.prototype.get should return `undefined` for not-present entries, instead of `null` (#27)

Having said that, most platform APIs today return null for not-present cases, e.g., `getElementById()` or `getAttribute()`. And types here are constrained whereas they are not with `Map` and `Set`.

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https://github.com/whatwg/xhr/issues/27#issuecomment-145949463