Avoid NotAuthenticated critical exceptions in logs

Some devices make calls without specifiying credentials, internally needed by API calls.
This results in the following error trace in logs:

[2019-07-23 12:48:46] dav-server.CRITICAL: WT\DAV\Exception\NotAuthenticated in /usr/share/webtop/webdav/lib/webtop/DAV/Connector/PrincipalBackend.php:74
Stack trace:
#0 /usr/share/webtop/webdav/vendor/sabre/dav/lib/DAVACL/AbstractPrincipalCollection.php(117): WT\DAV\Connector\PrincipalBackend->getPrincipalByPath('principals/gior...')
#1 /usr/share/webtop/webdav/vendor/sabre/dav/lib/DAV/Collection.php(54): Sabre\DAVACL\AbstractPrincipalCollection->getChild('giorgio@ecoclim...')
#2 /usr/share/webtop/webdav/vendor/sabre/dav/lib/DAV/Tree.php(105): Sabre\DAV\Collection->childExists('giorgio@ecoclim...')
#3 /usr/share/webtop/webdav/vendor/sabre/dav/lib/DAVACL/Plugin.php(878): Sabre\DAV\Tree->nodeExists('calendars/giorg...')
#4 [internal function]: Sabre\DAVACL\Plugin->beforeMethod(Object(Sabre\HTTP\Request), Object(Sabre\HTTP\Response))
#5 /usr/share/webtop/webdav/vendor/sabre/event/lib/EventEmitterTrait.php(105): call_user_func_array(Array, Array)
#6 /usr/share/webtop/webdav/vendor/sabre/dav/lib/DAV/Server.php(466): Sabre\Event\EventEmitter->emit('beforeMethod', Array)
#7 /usr/share/webtop/webdav/vendor/sabre/dav/lib/DAV/Server.php(254): Sabre\DAV\Server->invokeMethod(Object(Sabre\HTTP\Request), Object(Sabre\HTTP\Response))
#8 /usr/share/webtop/webdav/lib/webtop/DAV/Server.php(89): Sabre\DAV\Server->exec()
#9 /usr/share/webtop/webdav/server.php(21): WT\DAV\Server->exec()
#10 {main} [] []

It seems that this behaviour can be avoided by disabling support for unauthenticated access (http://sabre.io/dav/upgrade/3.1-to-3.2/)