Return 0 if element does not exists
Return 0 if element does not exists

This can return an empty result set in which case this spams the error log every second with a horrible amount of errors such as the following:

> Undefined index: num_participants at /media/psf/stable9/apps/spreed/lib/Room.php#195

Signed-off-by: Lukas Reschke <65adda4326914576405c9e3a62f4904d96573863@statuscode.ch>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed