v4: mark useEventHook param as not optional

[rtbo]

useEventHook param is currently marked optional.
This disables the possibility to destructure the query/mutation results directly in the callback parameter.
For example:

const { onResult } = useQuery(document);
onResult(({ data: { queryField } }) => {
    // use queryField
});

would currently yield a typescript error:

Property 'data' does not exist on type 'ApolloQueryResult<any> | undefined'

(note the | undefined also the event is declared as const resultEvent = useEventHook<ApolloQueryResult<any>>())

This PR solves the issue.

It is safe because:

• it only affects the type system, not the actual JS code
• nowhere in the source tree trigger is called with an undefined value (at least not in a way allowed by the type system)
• a possibly undefined result can still be expressed if necessary, but it has to be explicit: useEventHook<Param | undefined>()