YARN-10466.Fix NullPointerException in yarn-services Component.java. Contributed by D M Murali Krishna Reddy

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-applications/hadoop-yarn-services/hadoop-yarn-services-core/src/main/java/org/apache/hadoop/yarn/service/component/Component.java

@@ -830,6 +830,11 @@ public class Component implements EventHandler<ComponentEvent> {
                 targetExpressions.toArray(new TargetExpression[0])).build();
             break;
           }
+          if (constraint == null) {
+            LOG.info("[COMPONENT {}] Placement constraint: null ",
+                componentSpec.getName());
+            continue;
+          }
           // The default AND-ed final composite constraint
           if (finalConstraint != null) {
             finalConstraint = PlacementConstraints