MDL-77953 pagelib: Ensure that null $SCRIPT continues behaving the same

It's possible to have some Moodle components soft linked instead
of being real directories within codebase (within dirroot).

For example, Composer's "vendor" directory can be soft linked
(from elsewhere), or also plugins can be installed using soft
links.

In those cases, Moodle calculates the $SCRIPT global as null. And,
then, string operations on it are emitting a PHP deprecation message
with PHP 8.1 and up.

This fix just ensures that the behaviour is the same than before
PHP 8.1, aka: ltrim(null) = '' (empty string), without any PHP warning.

lib/pagelib.php

@@ -1873,7 +1873,7 @@ class moodle_page {
         }
 
         if (is_null($script)) {
-            $script = ltrim($SCRIPT, '/');
+            $script = ltrim($SCRIPT ?? '', '/');
             $len = strlen($CFG->admin);
             if (substr($script, 0, $len) == $CFG->admin) {
                 $script = 'admin' . substr($script, $len);