Hotfix: 채팅 조회 API 해당하는 채팅 없을 때 빈 리스트 반환

src/main/java/com/kuit/chatdiary/service/ChatService.java

@@ -10,6 +10,7 @@
 import org.springframework.http.HttpStatus;
 import org.springframework.stereotype.Service;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -74,7 +75,7 @@ public List<ChatGetResponseDTO> getChats(Long userId, Long lastChatId) {
         List<Chat> chats = chatRepository.findTop10ByUserIdAndChatIdGreaterThanOrderByChatIdDesc(userId, lastChatId);
         System.out.println(chats);
         if (chats.isEmpty()) {
-            return null;
+            return new ArrayList<>();
         }
         return chats.stream()
                 .map(ChatGetResponseDTO::new)