when id_token_hint is not JWT, CIBA authorization request returns HTTP 500

Bug description

when id_token_hint is not JWT, CIBA authorization request returns HTTP 500

When I use `id_token_hint` without JWT value, CIBA authorization request returns HTTP 500

not right number of dots, 1

When I use `id_token_hint` without JWT value, but with right number of dots, there is error HTTP 500

`Failed to parse json: Unrecognized token 'u': was expecting ('true', 'false' or 'null')\n at [Source: (String)\"u�\"; line: 1, column: 2]"`

How to reproduce the issue

1. follow steps in OPENAM-15049 without D
2. create authentication tree "test" with nodes "Start" -> "Username collector" -> "Success"
3. configure OAuth2Provider, Advanced OpenId Connect, OpenID Connect acr_values to Auth Chain Mapping - add key=push value=test
4. change "login_hint":"demo" to "id_token_hint":"demo" or to "id_token_hint":"demo.demo.demo" in JWT, save encoded JWT to file
5. do request as in OPENAM-15049 step C

Expected behaviour

HTTP 400 with incorrect JWT in id_token_hint message

Current behaviour

HTTP 500 with messages above