The bmx6-info script will return null when no tunnels are known to BMX6.
However, the "Graph" and the "Tunnels" pages expect a JSON structure with an
array of tunnels. In case no tunnels are known to BMX6, bmx6-info now returns
'{ "tunnels" : [] }'.
Fixes#434
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>