Call of jwt_sign() and saml_sso_url() panics

[alex-schneider]

Describe the bug
Call of jwt_sign() and saml_sso_url() w/o or incomplete number of params panics.

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? Run couper version or docker run avenga/couper version
0 2021-05-25 dev

2. Provide your configuration file *.hcl. Remove sensitive data.

server "jwt" {
  endpoint "/" {
    response {
      body = jwt_sign()
    }
  }
}

3. Provide a curl call for reproduction
   3.1 You may use your browsers developer console and hit copy as curl.

curl -v 'http://localhost:8080/'

Expected behavior
No panic, better error log messages.