Fix install failure in 003_Permission_system_upgrade.php (issue #760)

My version of MySQL "forgave" inserting NULL in what turned out to be a
NOT NULL varchar, converting it to the empty string.  But I didn't mean
to do that, and it breaks elsewhere with the error

"Field 'description' doesn't have a default value".
003_Permission_system_upgrade.php: status field shouldn't permit NULLs

It has a sensible default value anyway, so this shouldn't cause any
difficulty.