fix(next): unhandled error in renderListView server function when no user present (#14878)

Previously, the following error may be thrown when no `user` is present
on the `req`:

```ts
 TypeError: Cannot read properties of null (reading 'collection')
    at renderListHandler (packages/next/src/views/List/handleServerFunction.tsx:66:28)
    at async Server.<anonymous> (test/dev.ts:129:5)
  64 |           {
  65 |             'user.relationTo': {
> 66 |               equals: user.collection,
     |                            ^
  67 |             },
  68 |           },
  69 |           { {
  digest: '4059983158'
}
 POST /admin/collections/with-list-drawer 500 in 164ms (compile: 2ms, render: 162ms)
```

I'm seeing it thrown a lot in our test suites, likely because it's
logging out the user while a previous request is still going on.

This PR explicitly throws an `UnauthorizedError` before the server
function tries to access properties on the `user` object.