Handle null/undefined JsArrays as empty in Natives.asList()

Fix a resulting bug in PublishCommentScreen causing an error when
nobody has voted on a particular label.

Change-Id: I0c6bfbfa4ea151df4e9b588251fd1895745a7edb

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/PublishCommentScreen.java

@@ -267,10 +267,12 @@ public class PublishCommentScreen extends AccountScreen implements
     vp.setStyleName(Gerrit.RESOURCES.css().approvalCategoryList());
 
     Short prior = null;
-    for (ApprovalInfo app : Natives.asList(label.all())) {
-      if (app._account_id() == Gerrit.getUserAccount().getId().get()) {
-        prior = app.value();
-        break;
+    if (label.all() != null) {
+      for (ApprovalInfo app : Natives.asList(label.all())) {
+        if (app._account_id() == Gerrit.getUserAccount().getId().get()) {
+          prior = app.value();
+          break;
+        }
       }
     }
 

gerrit-gwtui/src/main/java/com/google/gerrit/client/rpc/Natives.java

@@ -37,6 +37,9 @@ public class Natives {
 
   public static <T extends JavaScriptObject> List<T> asList(
       final JsArray<T> arr) {
+    if (arr == null) {
+      return null;
+    }
     return new AbstractList<T>() {
       @Override
       public T set(int index, T element) {