On one of our sites we have disabled all activity modules.

Unfortunately that causes the 'cron not run recently' warning to occur as it does:

$lastcron = $DB->get_field_sql('SELECT MAX(lastcron) FROM {modules}');

Perhaps we could use scheduled tasks lastrun for this indicator instead.

Steps to reproduce:

1. Install a new Moodle dont setup cron
2. Disable all activity modules
3. Run cron manally

Expected result:
The 'cron.php maintenance script</a> has not been run for at least 24 hours.' does not appear.

Actual result:
The cron warning appears and never disappears.