如何在AssertJ中检查相等或两者都为空？

import org.assertj.core.api.Assertions;

public class ExampleTest {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";
        String str3 = null;
        String str4 = null;

        // 检查相等
        Assertions.assertThat(str1).isEqualTo("Hello");
        Assertions.assertThat(str2).isEqualTo("World");

        // 检查为空
        Assertions.assertThat(str3).isNull();
        Assertions.assertThat(str4).isNull();
    }
}