NullPointerException: element cannot be mapped to a null key

java.lang.NullPointerException: element cannot be mapped to a null key

import java.util.ArrayList;
import java.util.stream.Collectors;
import lombok.Builder;
import lombok.Data;

@Data
@Builder
public class S {

    private String a;
    private String b;

    public static void main(String[] args) {
        ArrayList<S> list = new ArrayList<>();
        list.add(S.builder().a(null).b("1").build()); // groupingBy的元素为空
        list.add(S.builder().a(null).b("2").build());// groupingBy的元素为空
        list.add(S.builder().a("a").b("1").build());
        Map<String, List<S>> result = list.stream().collect(Collectors.groupingBy(S::getA)); // 此行代码报错
    }
}

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;
import lombok.Builder;
import lombok.Data;

@Data
@Builder
public class S {

    private String a;
    private String b;

    public static void main(String[] args) {
        ArrayList<S> list = new ArrayList<>();
        list.add(S.builder().a(null).b("1").build());
        list.add(S.builder().a(null).b("2").build());
        list.add(S.builder().a("a").b("1").build());
        Map<Optional<String>, List<S>> aOptionalMap = list.stream().collect(Collectors.groupingBy(s -> Optional.ofNullable(s.getA()))); 
        Map<String, List<S>> result = new HashMap<>();
        aOptionalMap.keySet().parallelStream().forEach(aOptional ->
            result.put(aOptional.orElse(null), aOptionalMap.get(aOptional)));
    }
}