return not found page if no serverData or if page is draft

[mikkelklokkerud]

Currently there are two issues related to routing of open position pages:

• Visitors get "internal server error" when trying to visit a route that doesn't exist. With this PR it returns our custom "not found page" instead
• Draft sanity documents don't get filtered out. With this PR they're filtered out and not published

[klyngen]

I think this solution works, but only for jobbe-i-alv.
Put the same component into a page/500.ts-file. See the documentation below
https://www.gatsbyjs.com/docs/how-to/adding-common-features/add-500-page/