ONOS-3356 Fixed NPE when processing configuration of a non-existent device.

Change-Id: I00369a67c406634b2ee7d379cad6e0fc41842388
diff --git a/core/net/src/main/java/org/onosproject/net/device/impl/BasicDeviceOperator.java b/core/net/src/main/java/org/onosproject/net/device/impl/BasicDeviceOperator.java
index fa90eb6..3240342 100644
--- a/core/net/src/main/java/org/onosproject/net/device/impl/BasicDeviceOperator.java
+++ b/core/net/src/main/java/org/onosproject/net/device/impl/BasicDeviceOperator.java
@@ -51,7 +51,7 @@
      * @return DeviceDescription based on both sources
      */
     public static DeviceDescription combine(BasicDeviceConfig bdc, DeviceDescription descr) {
-        if (bdc == null) {
+        if (bdc == null || descr == null) {
             return descr;
         }