fix: filter null references for detail responses

eno-core/src/main/java/fr/insee/eno/core/processing/in/steps/ddi/DDIInsertDetailResponses.java

@@ -65,6 +65,7 @@ private void resolveDetailResponses(SimpleMultipleChoiceQuestion simpleMultipleC
                 .stream()
                 .filter(ModalityAttachment.CodeAttachment.class::isInstance)
                 .map(ModalityAttachment.CodeAttachment.class::cast)
+                .filter(codeAttachment -> codeAttachment.getAttachmentBase() != null)
                 .collect(Collectors.toMap(
                         ModalityAttachment.CodeAttachment::getAttachmentBase, codeAttachment -> codeAttachment));
         //