Prevent NPE in ListTasks

If the show-queue command is executed while an ls-projects command is
executing, NPE occurs in the ListTasks REST API endpoint.

Add a null check to prevent this.

Bug: Issue 3211
Change-Id: Iea0ad5d1336d258ee2cd898eb5488ef610eb7d0c

gerrit-server/src/main/java/com/google/gerrit/server/config/ListTasks.java

@@ -124,7 +124,10 @@ public class ListTasks implements RestReadView<ConfigResource> {
 
       if (task instanceof ProjectTask) {
         ProjectTask<?> projectTask = ((ProjectTask<?>) task);
-        this.projectName = projectTask.getProjectNameKey().get();
+        Project.NameKey name = projectTask.getProjectNameKey();
+        if (name != null) {
+          this.projectName = name.get();
+        }
         this.remoteName = projectTask.getRemoteName();
       }
     }