[Auth] Authorization failure when engaging empty scopes for HTTP service config

[ldclakmal]

Description:
If the scopes of HTTP service config is defined as [] (empty scopes), there is no way to make the requests "Authorization Successfull".

@http:ServiceConfig {
    basePath: "/hello",
    authConfig: {
        authentication: { enabled: true },
        scopes: []
    }
}

Steps to reproduce:

sample.bal

import ballerina/http;

http:AuthProvider basicAuthProvider = {
    scheme: "basic",
    authStoreProvider: "config"
};

listener http:Listener ep = new(9090, config = {
    authProviders: [basicAuthProvider],
    secureSocket: {
        keyStore: {
            path: "${ballerina.home}/bre/security/ballerinaKeystore.p12",
            password: "ballerina"
        }
    }
});

@http:ServiceConfig {
    authConfig: {
        authentication: { enabled: true },
        scopes: []
    }
}
service hello on ep {
    resource function sayHello(http:Caller caller, http:Request req) {
        _ = caller->respond("Hello, World!!!");
    }
}

users.toml

["b7a.users"]
["b7a.users.tom"]
password="123"

Run the program - $ ballerina run -c users.toml sample.bal
Test the program - $ curl -k -u tom:123 https://localhost:9090/hello/sayHello

Expected output - Hello, World!!!
Actual output - Authorization failure

Affected Versions:
0.990.2

OS, DB, other environment details and versions:
Ubunut 16.04