refactor: 우수 스터디원이 없으면 null 반환

src/main/java/com/gdschongik/gdsc/domain/study/dto/response/StudentMyCompleteStudyResponse.java

@@ -5,7 +5,7 @@
 import com.gdschongik.gdsc.domain.study.domain.StudyHistory;
 import com.gdschongik.gdsc.domain.study.domain.StudyHistoryStatus;
 import io.swagger.v3.oas.annotations.media.Schema;
-import java.util.ArrayList;
+import jakarta.annotation.Nullable;
 import java.util.List;
 
 public record StudentMyCompleteStudyResponse(
@@ -19,7 +19,7 @@ public record StudentMyCompleteStudyResponse(
         @Schema(description = "멘토 이름") String mentorName,
         @Schema(description = "총 주차수") Long totalWeek,
         @Schema(description = "수료 여부") StudyHistoryStatus studyHistoryStatus,
-        @Schema(description = "우수 스터디원 여부") List<AchievementType> achievements) {
+        @Nullable @Schema(description = "우수 스터디원 여부") List<AchievementType> achievements) {
 
     public static StudentMyCompleteStudyResponse of(StudyHistory studyHistory, List<AchievementType> achievements) {
         return new StudentMyCompleteStudyResponse(
@@ -33,6 +33,6 @@ public static StudentMyCompleteStudyResponse of(StudyHistory studyHistory, List<
                 studyHistory.getStudy().getMentor().getName(),
                 studyHistory.getStudy().getTotalWeek(),
                 studyHistory.getStudyHistoryStatus(),
-                (achievements == null) ? new ArrayList<>() : achievements);
+                achievements);
     }
 }