MDL-69758 Web services: core_get_course_contents PHP warnings

Calls to core_get_course_contents can cause PHP warnings in the server
log if there are any URL instances in the course with sketchy URLs.
This is because url_export_contents incorrectly returns null instead
of an empty array if there is an error.

mod/url/lib.php

@@ -274,7 +274,7 @@ function url_export_contents($cm, $baseurl) {
     $fullurl = str_replace('&', '&', url_get_full_url($urlrecord, $cm, $course));
     $isurl = clean_param($fullurl, PARAM_URL);
     if (empty($isurl)) {
-        return null;
+        return [];
     }
 
     $url = array();