MDL-70815 completion: Cast custom data to array

Since cm_info::customdata can be of any type, we need to cast it to an
array first before checking for custom completion rules. Otherwise,
an exception can be thrown (e.g. customdata has been set as an stdClass)

lib/completionlib.php

@@ -1107,8 +1107,10 @@ class completion_info {
 
         // Custom activity module completion data.
 
+        // Cast custom data to array before checking for custom completion rules.
+        $customdata = (array)$cm->customdata;
         // Return early if the plugin does not define custom completion rules.
-        if (empty($cm->customdata['customcompletionrules'])) {
+        if (empty($customdata['customcompletionrules'])) {
             return [];
         }
 
@@ -1120,7 +1122,7 @@ class completion_info {
 
         /** @var activity_custom_completion $customcmcompletion */
         $customcmcompletion = new $cmcompletionclass($cm, $userid);
-        foreach ($cm->customdata['customcompletionrules'] as $rule => $enabled) {
+        foreach ($customdata['customcompletionrules'] as $rule => $enabled) {
             if (!$enabled) {
                 // Skip inactive completion rules.
                 continue;