luci-app-bmx6: Fix corner case in bmx6-info?tunnels

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>

luci-app-bmx6/bmx6/www/cgi-bin/bmx6-info

@@ -100,7 +100,12 @@ if [ "$QUERY" == 'neighbours' ]; then
 fi
 
 if [ "$QUERY" == 'tunnels' ]; then
-  bmx6 -c --jshow tunnels /r=0
+  tunnels=$(bmx6 -c --jshow tunnels /r=0)
+  if [ -z $tunnels ]; then
+    echo '{ "tunnels" : [] }'
+  else
+    echo $tunnels
+  fi
   exit 0
 fi