[heycam/webidl] Dictionary types being nullable doesn't make sense (#967)

Per <https://heycam.github.io/webidl/#idl-nullable-type>, dictionary types are allowed to be nullable (they're not excluded by any of the conditions), but *unions* containing dictionary types can't be nullable.

That is, `MyDict?` is valid, but `(MyDict or DOMString)?` is not.

This doesn't seem to make sense. Per <https://heycam.github.io/webidl/#es-dictionary>, JS `null` is *always* acceptable to convert to a dictionary type; nullability doesn't enter into the equation at all. In other words, dictionary types are *implicitly nullable*. You can also see this more easily in <https://heycam.github.io/webidl/#es-union>, where both `null` and `undefined` can be turned into a dictionary type if a union includes one.

Tracing the blame back, this change occurred in <https://github.com/heycam/webidl/commit/6ac261bd550ce99d62c54ccf831b4e0ab7dee5df>, with the reasoning given being "Disallow dictionaries from being nullable only as operation argument types.". But that edit preserved the fact that JS `null` is *always* convertable to a dictionary type, so I'm not sure what it was actually trying to do.

I suggest that we revert that change and just always disallow dictionary types from being nullable (since they're always implicitly nullable already).

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