fix bug

cqb-paper/src/main/java/com/qmth/cqb/paper/assemble/SubQuestionDtoAssembler.java

@@ -6,6 +6,7 @@ import com.qmth.cqb.question.model.QuesOption;
 import com.qmth.cqb.question.model.Question;
 import org.springframework.stereotype.Component;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -35,12 +36,18 @@ public class SubQuestionDtoAssembler {
     }
 
     public List<SubQuestionDto> toDtoList(List<Question> questionList){
+        if(questionList == null || questionList.size() == 0 ){
+            return new ArrayList<SubQuestionDto>();
+        }
         return questionList.stream()
                 .map(question -> toDto(question))
                 .collect(Collectors.toList());
     }
 
     public List<QuesOptionDto> toOptionDtoList(List<QuesOption> quesOptionList){
+        if(quesOptionList == null || quesOptionList.size() == 0 ){
+            return new ArrayList<QuesOptionDto>();
+        }
         return quesOptionList.stream()
                 .map(quesOption -> toOptionDto(quesOption))
                 .collect(Collectors.toList());