Return empty system addressbook since we cannot easily limit it to groups

Signed-off-by: Julius Härtl <jus@bitgrid.net>

apps/dav/lib/CardDAV/SystemAddressbook.php

@@ -40,7 +40,9 @@ class SystemAddressbook extends AddressBook {
 	}
 
 	public function getChildren() {
-		if ($this->config->getAppValue('core', 'shareapi_allow_share_dialog_user_enumeration', 'yes') !== 'yes') {
+		$shareEnumeration = $this->config->getAppValue('core', 'shareapi_allow_share_dialog_user_enumeration', 'yes') === 'yes';
+		$restrictShareEnumeration = $this->config->getAppValue('core', 'shareapi_restrict_user_enumeration_to_group', 'yes') === 'yes';
+		if (!$shareEnumeration || ($shareEnumeration && $restrictShareEnumeration)) {
 			return [];
 		}