Test item GetAssertionEmptyUserIdTest

[nuno0529]

{
  "description": "Tests if empty user IDs are omitted in the response.",
  "error_message": "Cannot make credential with an empty user ID.",
  "id": "get_assertion_empty_user_id",
  "observations": [
    "A prompt was expected, but not performed. Sometimes it is just not recognized if performed too fast.",
    "The failing error code is `CTAP2_ERR_PROCESSING`."
  ],
  "result": "fail",
  "tags": []
},

https://github.com/fido-alliance/fido-2-specs/pull/963 Although fido2 spec says empty account identifier is valid but can't be returned in get(), this case makes authenticator need to save some useless data...

Supplement on Nov-25 I can't find the string to say but can't be returned in get() in spec.