…-existent users

…undefined` values

The `EncryptedState` cookie gets quite large due to the nature of encryption and addition verification tags and signing.

In order to reduce the size of the cookie we set we have done two things.

1. For the Okta IDX API, it turns out we only need the first part of the `stateHandle` to persist. The API continues to work with just this bit.
  - The `stateHandle` string is 817 characters long, however only the first part before the `~` is needed, which is just 46 characters in length.
  - This is actually the same bit that is required for the `stateToken` parameter in the `/login/token/redirect` endpoint used after authentication
    - see: https://github.com/guardian/gateway/blob/34311cdf8524f247a11841fbf7bdf9568f5829ea/src/server/lib/okta/idx/shared/idxFetch.ts#L128
2. We remove `undefined` keys and values, this means we dont have to persist the key and undefined value in the encrypted JSON, thus saving space.