fix : 미션 인증 시 commentNum 디폴트값 지정

Author: seungueonn

src/main/java/com/moing/backend/domain/missionArchive/application/mapper/MissionArchiveMapper.java

@@ -29,6 +29,7 @@ public static MissionArchive mapToMissionArchive(MissionArchiveReq missionArchiv
                 .mission(mission)
                 .heartList(new ArrayList<>())
                 .contents(missionArchiveReq.getContents())
+                .commentNum(0L)
                 .build();
     }