fix https://linear.app/tryghost/issue/SLO-88/typeerror-cannot-read-properties-of-null-reading-relations
- in the event that we make it through the version mismatch code, but
without a key, which is possible if you send a request like POST
/ghost/api/v2/content/posts/`, then the version mismatch code will try
and look up the API key attached to a null key, which won't work
- we should handle this case and soft return, to avoid trying to read
`.relations` from `null`
- I'm not entirely convinced by how this code works in general, it seems
quite confusing to reason about, but this commit should solve the HTTP
500 we've been seeing from this
- perhaps in the future we can return earlier in the flow if we receive
a `null` key
6c7b230efe