Settings cannot be overriden when $name not specified in getItem() call

In CRM_Core_BAO_Setting::getItem(), the second parameter, $name, is optional. When not present, and array containing all settings for the given $group is returned.

And in this case, the settings override through the $civicrm_setting variable does not work - see CRM_Core_BAO_Setting::getOverride:
if ($group && $name && isset($civicrm_setting[$group][$name]))

There is some extra code dealing with this at the bottom of the getItem() function, but it presupposes that the setting already exists in the database - which is certainly not the case for non-core items - mostly extensions.

When no $name is present, the getItem() function should return an array consisting of the sum of all override settings + all settings present in the database for the given $group (with the overrides taking precedence, and applying even if the setting is not defined in the database).