fix(CardDAV): allow disabling of the system address book via config option

Signed-off-by: Anna Larch <anna@nextcloud.com>

apps/dav/lib/CardDAV/UserAddressBooks.php

@@ -88,7 +88,8 @@ class UserAddressBooks extends \Sabre\CardDAV\AddressBookHome {
 		$addressBooks = $this->carddavBackend->getAddressBooksForUser($this->principalUri);
 		// add the system address book
 		$systemAddressBook = null;
-		if (is_string($principal) && $principal !== 'principals/system/system' && $this->carddavBackend instanceof CardDavBackend) {
+		$systemAddressBookExposed = $this->config->getAppValue('dav', 'system_addressbook_exposed', 'yes') === 'yes';
+		if ($systemAddressBookExposed && is_string($principal) && $principal !== 'principals/system/system' && $this->carddavBackend instanceof CardDavBackend) {
 			$systemAddressBook = $this->carddavBackend->getAddressBooksByUri('principals/system/system', 'system');
 			if ($systemAddressBook !== null) {
 				$systemAddressBook['uri'] = SystemAddressbook::URI_SHARED;