Does your plugin's PLUGINNAME_supports() function return true for FEATURE_MOD_INTRO? Because if it does, then it is your own code that is making this field mandatory ...
If it's not that, then I suggest you make sure Debugging is set to developer, then check the code in the indicated part of modlib.php.
If it's not that, then I suggest you make sure Debugging is set to developer, then check the code in the indicated part of modlib.php.