Fixed a potential NPE in link discovery.

Change-Id: I3bff307e54a1e3bdac1053dfea6e96ae3cd29f07

@@ -40,6 +40,7 @@ import org.slf4j.Logger;
 import java.nio.ByteBuffer;
 import java.util.Set;
 
+import static com.google.common.base.Strings.isNullOrEmpty;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static org.onosproject.net.PortNumber.portNumber;
 import static org.onosproject.net.flow.DefaultTrafficTreatment.builder;
@@ -165,7 +166,10 @@ public class LinkDiscovery implements TimerTask {
 
             PortNumber srcPort = portNumber(onoslldp.getPort());
             PortNumber dstPort = packetContext.inPacket().receivedFrom().port();
-            DeviceId srcDeviceId = DeviceId.deviceId(onoslldp.getDeviceString());
+
+            String idString = onoslldp.getDeviceString();
+            if (!isNullOrEmpty(idString)) {
+                DeviceId srcDeviceId = DeviceId.deviceId(idString);
                 DeviceId dstDeviceId = packetContext.inPacket().receivedFrom().deviceId();
 
                 ConnectPoint src = new ConnectPoint(srcDeviceId, srcPort);
@@ -180,6 +184,7 @@ public class LinkDiscovery implements TimerTask {
                 }
                 return true;
             }
+        }
         return false;
     }