When using the `org.keycloak.adapters.servlet.KeycloakOIDCFilter` a `NullPointerException`
can be thrown in the `org.keycloak.adapters.servlet.FilterSessionStore` within the `getParam`
method of the generated wrapper in `buildWrapper` when the `content-type` is not set.
Since the `content-type` is only used to parse the body. We just check whether the `body`
is `null` and if so avoid touching the `content-type` which prevents the NPE.
If the `body` is null we return an empty `MultivaluedHashMap` for the parameters.
