fix: 답변이 null인 경우에 대한 처리 추가 (#700)

backend/ddang/src/main/java/com/ddang/ddang/auction/presentation/dto/response/ReadQnaResponse.java

@@ -16,7 +16,7 @@ public static ReadQnaResponse from(final ReadQnaDto readQnaDto) {
     }
 
     private static ReadAnswerResponse processReadAnswerResponse(final ReadAnswerDto readAnswerDto) {
-        if (readAnswerDto.isDeleted()) {
+        if (readAnswerDto == null || readAnswerDto.isDeleted()) {
             return null;
         }