`useRouteParam` should return `string | undefined` instead of just `string`

[znicholasbrown]

I think we're relying on the type coming from useRoute so I might need to transfer this issue but a call to useRouteParam returns a type of string when in fact it can be undefined.

[pleek91]

I really want to do this and make the types more similar to useRouteQueryParam. And maybe account for the fact that vue-router changes the param's value before it unmounts the component.

BUTTTT changing the types produced 132 errors in nebula-ui... So I don't think I'm going to tackle this right now.