Checking invalid post crashes server

[xHomu]

The way the post fetches work atm would crash the server due to graphql type errors:

https://github.com/manawiki/core/blob/main/app/routes/_site+/p+/utils/fetchPostWithSlug.server.ts

• invariant(post, "Post doesn't exist");

https://github.com/manawiki/core/blob/main/app/routes/_site+/p+/utils/fetchPostComments.server.ts

• postParentId: "JSON!", this triggers graphql type error

Because these graphql queries are dependent each other, one's invalid would trigger a server crash onto the other. We should rework these queries a bit safer so it would just render a Post not found.