The default value is an empty string (""), which in PHP evaluates to 0 when
cast to a string in our DML layer.
The fix is to forcibly make it null which is not cast to string in DML
for optional strings.
Add field 'visibleoncoursepage' to the course_modules table
Add site-wide setting for turning on stealth mode availability
Add callback for "stealth" mode support in the course formats
Change display of modules/sections availability on the course page
Andrew Nicols