History 'next' returns undefined in js not null

[tomczoink]

The docs () currently state if there is no next page, null is returned. (https://github.com/ably/docs/blame/main/content/partials/types/_paginated_result.textile#L81)

Currently undefined is actually returned (as in ts void is returned, https://github.com/ably/ably-js/blob/3386abae8f35f5d41f016c0feeb7406bb5537cb2/src/common/lib/client/paginatedresource.ts#L178).

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