CLOUDSTACK-5453: Fix VPN connection monitoring in KVM

It's broken by:

commit 7f104b1feb
Author: Edison Su <sudison@gmail.com>
Date: Fri Oct 12 18:24:50 2012 -0700

    [PATCH] CLOUDSTACK-335: fix lb for vpc on kvm

core/src/com/cloud/agent/resource/virtualnetwork/VirtualRoutingResource.java

@@ -667,7 +667,7 @@ public class VirtualRoutingResource implements Manager {
         }
 
         final String result = routerProxy("checkbatchs2svpn.sh", routerIP, args);
-        if (result != null) {
+        if (result == null || result.isEmpty()) {
             return new CheckS2SVpnConnectionsAnswer(cmd, false, "CheckS2SVpnConneciontsCommand failed");
         }
         return new CheckS2SVpnConnectionsAnswer(cmd, true, result);