[w3c/uievents] InputEvent.data should be nullable (#116)

@garykac 

According to the usage in [InputEvent spec](https://w3c.github.io/input-events/#h-overview) `InputEvent.data` should be nullable. ([Link](https://w3c.github.io/uievents/#interface-inputevent) to the definition in UIEvent)

The reasons are:
1. `InputEvent.data` is not inherited from `UIEvent`, so it's OK to make it nullable;
2. `InputEvent` relies on the null check to tell JS which attribute they should use - `data` or `dataTransfer`;
3. We can't use `data == ""` in this case because:
    * There are other `inputType`s (e.g. `'formatBold'`) that shouldn't have a `data` field, and having `data == ""` would make it confusing;
    * It's possible for an IME to fire an `InputEvent` with empty text, which should still be valid (Not 100% sure if any browser is actually doing this though)

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