MDL-71119 core_badges: Initialise scopes param in backpack-connect.php

The scopes parameter should be passed when creating the OAuth2 badges
client. As it is an optional parameter, when it's empty, it will
be initilised with the supported scopes for the backpack issuer.

It will happen, for instance, when a call is done to oauth2callback.php
because, as defined in RFC6749, OAuth2 authorization response only
supports code and state.

badges/backpack-connect.php

@@ -43,6 +43,10 @@
     $returnurl = new moodle_url('/badges/backpack-connect.php',
         ['action' => 'authorization', 'sesskey' => sesskey(), 'backpackid' => $backpackid]);
 
+    // If scope is not passed as parameter, use the issuer supported scopes.
+    if (empty($scope)) {
+        $scope = $issuer->get('scopessupported');
+    }
     $client = new core_badges\oauth2\client($issuer, $returnurl, $scope, $externalbackpack);
     if ($client) {
         if (!$client->is_logged_in()) {