Defensive null handling

Always handle null references and values defensively to prevent NullPointerExceptions and unexpected behavior. Follow these practices:

1. Check nulls before dereferencing: Always verify a reference is not null before accessing its methods or properties: ```java // Instead of directly accessing possibly null objects Engine engine = shard.getEngineOrNull(); assertThat(engine != null && engine.isThrottled(), equalTo(true));

// Or for parsing operations if (token == null) { token = parser.nextToken(); }

2. **Use APIs that handle nulls gracefully**: Prefer methods that allow specifying default values when parsing or converting:
```java
// Instead of Boolean.parseBoolean which doesn't handle nulls well
assertThat(Booleans.parseBoolean((String) indexSettings.get(setting.getKey()), false), matcher);

1. Defensive coding for asynchronous operations: When objects might be deleted or changed by other threads:

   var project = clusterService.state().metadata().projects().get(projectId);
   PersistentTasksCustomMetadata.PersistentTask<?> persistentTask = 
    project == null ? null : PersistentTasksCustomMetadata.getTaskWithId(project, getPersistentTask());
   

These patterns help create more robust code that can handle edge cases and prevent crashes in production.