API boundary null handling

Establish consistent null handling patterns at API boundaries to prevent null pointer exceptions and improve code clarity:

1. Validate method parameters using explicit null checks:

   public void setHttpClient(HttpClient httpClient) {
    Assert.notNull(httpClient, "HttpClient must not be null");
    this.httpClient = httpClient;
   }
   

2. Return empty collections instead of null:

   public List<PropertyAccessor> getPropertyAccessors() {
    return Collections.emptyList();  // Instead of returning null
   }
   

3. Use Optional only as a return type, never as a field: ```java // DON’T private Optional errorStatus; // Avoid Optional as field

// DO public Optional findUserById(String id) { // OK as return type // ... }

4. For nullable return values in internal APIs, prefer explicit @Nullable annotation over Optional:
```java
@Nullable
protected Class<?> getReturnType(Method method) {
    // ...
}

These patterns ensure consistent null handling, improve code readability, and reduce the likelihood of null-related bugs. They also help maintain clear contracts between components while avoiding unnecessary Optional usage.