Modules execution plan: fix NPE (prebid#3902)

Author: CTMBNara

src/main/java/org/prebid/server/hooks/execution/HookStageExecutor.java

@@ -9,6 +9,7 @@
 import io.vertx.core.Vertx;
 import org.apache.commons.collections4.CollectionUtils;
 import org.apache.commons.collections4.ListUtils;
+import org.apache.commons.collections4.MapUtils;
 import org.apache.commons.collections4.map.DefaultedMap;
 import org.apache.commons.lang3.ObjectUtils;
 import org.apache.commons.lang3.StringUtils;
@@ -155,7 +156,7 @@ private static ExecutionPlan parseExecutionPlan(String executionPlan, JacksonMap
     }
 
     private static ExecutionPlan validateExecutionPlan(ExecutionPlan plan, HookCatalog hookCatalog) {
-        plan.getEndpoints().values().stream()
+        MapUtils.emptyIfNull(plan.getEndpoints()).values().stream()
                 .map(EndpointExecutionPlan::getStages)
                 .map(Map::entrySet)
                 .flatMap(Collection::stream)
@@ -412,8 +413,7 @@ private StageExecutionPlan effectiveStagePlanFrom(
     }
 
     private static StageExecutionPlan stagePlanFrom(ExecutionPlan executionPlan, Endpoint endpoint, Stage stage) {
-        return executionPlan
-                .getEndpoints()
+        return MapUtils.emptyIfNull(executionPlan.getEndpoints())
                 .getOrDefault(endpoint, EndpointExecutionPlan.empty())
                 .getStages()
                 .getOrDefault(stage, StageExecutionPlan.empty());

src/main/java/org/prebid/server/hooks/execution/model/ExecutionPlan.java

@@ -4,14 +4,13 @@
 import lombok.Value;
 import org.prebid.server.model.Endpoint;
 
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
 @Value(staticConstructor = "of")
 public class ExecutionPlan {
 
-    private static final ExecutionPlan EMPTY = of(null, Collections.emptyMap());
+    private static final ExecutionPlan EMPTY = of(null, null);
 
     @JsonProperty("abtests")
     List<ABTest> abTests;