refactor: 영수증 update record의 files에 @NotNull 추가

src/main/java/leets/weeth/domain/account/application/dto/ReceiptDTO.java

@@ -31,6 +31,6 @@ public record Update(
             @NotNull Integer amount,
             @NotNull LocalDate date,
             @NotNull Integer cardinal,
-            @Valid List<FileSaveRequest> files
+            @Valid List<@NotNull FileSaveRequest> files
     ) {}
 }