Prevent crash when asking a REPORT on (probably amongst others) a contact folder

This happens when a client also asks a REPORT for the contact folder as for the entries in it. From the logs:
EXCEPTION: <NSException: 0x7f964a245c70> NAME:NSInvalidArgumentException REASON:'_name' must not be an empty string INFO:(null)
Oct 31 12:40:25 sogod [33810]: <0x0x7f9649d30980[WOWatchDogChild]> child 33879 exited
Oct 31 12:40:25 sogod [33810]: <0x0x7f9649d30980[WOWatchDogChild]>  (terminated due to signal 6)

This occurs especially using clients (2 Android clients tested) that use draft-murchison-webdav-prefer-05 for the noroot hint instead of the microsoft extension implemented by SOGo (may be a patch coming up). So they also git the information of the root, expecting something to be there as the server gives this entry also as a reply.