Erroneous response on `direct_post`

[babisRoutis]

When handling a direct_post or a direct_post.jwt the endpoint returns one of the following responses:

• HTTP 200 (empty body)
• HTTP 200 with a json like

{ "redirect_uri" : "some uri" }

Depending on whether the verifier-ui initiated a transaction with a redirect_uri or not.

This difference causes problems on the wallet side

We should unify the response to always return a HTTP 200 with a JSON empty in the first case

{  }