MDL-67054 upgrade: prevent potential PHP notice

In some sites $CFG->requestcategoryselection may be not defined.

So this just wraps the use within empty, keeping the spirit intact,
aka, invert whatever the precious setting was storing.

null, 0, false, '0' => 1
1, true, '1' => 0

lib/db/upgrade.php

@@ -3614,7 +3614,7 @@ function xmldb_main_upgrade($oldversion) {
     if ($oldversion < 2019101600.01) {
 
         // Change the setting $CFG->requestcategoryselection into $CFG->lockrequestcategory with opposite value.
-        set_config('lockrequestcategory', !$CFG->requestcategoryselection);
+        set_config('lockrequestcategory', empty($CFG->requestcategoryselection));
 
         upgrade_main_savepoint(true, 2019101600.01);
     }