Prevents discoverLinksTasks from stopping if getLinks returns null (Exception)

Change-Id: I5b89380435e5283fd9f0c05da6c5c46e5e440bfa
diff --git a/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/LinkDiscoveryProvider.java b/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/LinkDiscoveryProvider.java
index a5fc042..7dc6f06 100644
--- a/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/LinkDiscoveryProvider.java
+++ b/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/LinkDiscoveryProvider.java
@@ -202,6 +202,10 @@
     }
 
     private void evaluateLinks(DeviceId deviceId, Set<LinkDescription> discoveredLinksDesc) {
+        if (discoveredLinksDesc == null) {
+            return;
+        }
+
         //The provider will get only existing links related to LinkDiscovery
         Set<Link> storedLinks = linkService.getDeviceEgressLinks(deviceId)
                 .stream()