Change default unset value to `undefined` instead of `null`.

[jeroenpelgrims]

Currently when a query param is not set it will get the value null. It would be better if we would return undefined instead since we can then more easily use standard TS optional notation. (question mark) This way we don't have to transform the value from null to undefined first.

e.g.:

function foo(value?: string) {
...
}

const param = useQueryParam(stringParam());

foo(param.value);