fix(ecs): Prevent NPE when a task def ARN is no longer in the cache (#…

…4338) (#4361)

Co-authored-by: Clare Liguori <liguori@amazon.com>
Co-authored-by: Eric Zimanyi <ezimanyi@google.com>

clouddriver-ecs/src/main/java/com/netflix/spinnaker/clouddriver/ecs/provider/agent/TaskDefinitionCachingAgent.java

@@ -149,7 +149,9 @@ private Set<TaskDefinition> retrieveFromCache(
     for (String taskDefArn : taskDefArns) {
       String key = Keys.getTaskDefinitionKey(accountName, region, taskDefArn);
       TaskDefinition taskDefinition = taskDefinitionCacheClient.get(key);
-      taskDefs.add(taskDefinition);
+      if (taskDefinition != null) {
+        taskDefs.add(taskDefinition);
+      }
     }
 
     return taskDefs;