Function input can be null

Change-Id: I9e9276f57c6c055935ed416c8ce197c24d2cfcc5

@@ -518,9 +518,13 @@ public class GossipLinkStore
     private final class LookupLink implements Function<LinkKey, Link> {
         @Override
         public Link apply(LinkKey input) {
+            if (input == null) {
+                return null;
+            } else {
                 return links.get(input);
             }
         }
+    }
 
     private void notifyDelegateIfNotNull(LinkEvent event) {
         if (event != null) {