Error while trying to authorize the request when scopes filed is not configured

[ldclakmal]

Description
When a HTTP service is configured with auth annotation is http:ServiceConfig, it should try to authenticate and authorize the inbound request. When the scopes field of the auth annotation is not provided, it should avoid the authorization phase. Currently it executes the authorization phase even though the scopes field is not provided. The logic should be fixed for that use case.

Steps to reproduce:

import ballerina/http;
listener http:Listener securedEP = new (9090, config = {
    secureSocket: {
        keyStore: {
            path: "/path/to/p12",
            password: "password"
        }
    }
});

@http:ServiceConfig {
    auth: [{
        jwtValidatorConfig: {
            issuer: "ballerina",
            audience: "vEwzbcasJVQm1jVYHUHCjhxZ4tYa",
            trustStoreConfig: {
                certificateAlias: "alias",
                trustStore: {
                    path: "/path/to/p12",
                    password: "password"
                }
            }
        }
    }]
}
service on securedEP {
    resource function get whoiam() returns @http:Payload {mediaType: "application/json"} json {
        return {"version": "v1"};
    }
}

curl https://localhost:9090/whoiam -k \
  -H 'Authorization: Bearer eyJhbGciOiJSUzI1NiIsICJ0eXAiOiJKV1QiLCAia2lkIjoiTlRBeFptTXhORE15WkRnM01UVTFaR00wTXpFek9ESmhaV0k0TkRObFpEVTFPR0ZrTmpGaU1RIn0.eyJpc3MiOiJiYWxsZXJpbmEiLCAic3ViIjoiYWRtaW4iLCAiYXVkIjoidkV3emJjYXNKVlFtMWpWWUhVSENqaHhaNHRZYSIsICJqdGkiOiJmMTJhYmE1Zi03MGQ2LTQ4OWUtODJkMi0yNTI1YzE1OGVkY2YiLCAiZXhwIjoxNjEzMjM0NDk1LCAibmJmIjoxNjEzMjMwODk1LCAiaWF0IjoxNjEzMjMwODk1fQ.Y5uu3BrQLDVIfkwR2b82Jv_KWpwYP2Wxtm22ufXKRV0A7mhZp0ljqYskBpJHNx6SR7D5Kxw7SOlhuU9BWQT_ZZNeHc2xIkZGZZyx3WwcAC8cFvM-XXJhkuRQyEdJ5uZtRl89ZbJ4PKLsc9_ENR5jgwfQNv3B1KOP_tG5KUtBt2VfIrfLcP3R9Pr6ZtYYA0AZxi7zB-lY6xT5H2OdecaSCNuykvsFL-pwfE5VP4E2v3ZukP79kT0hi6m4X0B93FKQBdkEHwGJVZW1P-JcQMybgfty9pDueAmWpTzZs5JaXsy32bB_DAIfEyYiKsS20e7Dr1NEOuAFpcaxgOwR7X6T6A'

[ballerina/http] started HTTPS/WSS listener 0.0.0.0:9090
error: {ballerina}TypeCastError {"message":"incompatible types: '()' cannot be cast to 'string|string[]'"}
	at ballerina.http.1_0_6:tryAuthenticate(auth_desugar.bal:78)
	   ballerina.http.1_0_6:authenticateResource(auth_desugar.bal:48)
	   kkeker.micro_bff.0_1_0.$anonType$_0:$get$whoiam(main.bal:21)

Affected Versions:
Ballerina SL Alpha2

Originally reported by Kirill Keker at Slack [1].

[1] https://ballerina-platform.slack.com/archives/C47EAELR1/p1613232067181500