Fix CDN advanced config validation

[annybs]

Advanced config (as raw JSON) cannot be submitted if it is missing cache.paths:

{
  "cache": {
    "paths": {}
  }
}

This can be an empty object, but it needs to be an object.

This may need to be fixed in https://github.com/edge/cache-config and then installed here as an npm update. If so, Stargate may also need the fix.

[annybs]

Misdiagnosed the error originally and went down the wrong rabbit hole. The proper fix can be found in the linked PR.