MDL-76356 various: avoid implicit conversion to arrays

PHP before version 8.1 automatically converted stdClass or 'false' to arrays if
function parameter expects array (for example, "reset").
PHP 8.1 shows notices in these situations

lib/completionlib.php

@@ -1055,7 +1055,7 @@ class completion_info {
             if (!isset($this->course->cacherev)) {
                 $this->course = get_course($this->course_id);
             }
-            if ($cacheddata = $completioncache->get($key)) {
+            if ($cacheddata = ($completioncache->get($key) ?: [])) {
                 if ($cacheddata['cacherev'] != $this->course->cacherev) {
                     // Course structure has been changed since the last caching, forget the cache.
                     $cacheddata = array();