fix: 修复多个 null 参数和空集合问题

1. EditReviewRepository - reviewedAt null 参数问题
   - PENDING 状态时 reviewedAt 为 null
   - 修复：null 时不在 SET 语句中包含该字段

2. KnowledgeItemApplicationService - 空 IN 子句问题
   - getKnowledgeManagementStatistics() 未检查空集合
   - 导致 WHERE id IN () MySQL 语法错误
   - 修复：添加空集合检查，提前返回零计数

验证：
- 其他 Neo4j 操作已正确处理 null 参数
- 其他 listByIds/removeByIds 调用已有保护

backend/services/data-management-service/src/main/java/com/datamate/datamanagement/application/KnowledgeItemApplicationService.java

@@ -266,6 +266,12 @@ public class KnowledgeItemApplicationService {
         response.setTotalKnowledgeSets(totalSets);
 
         List<String> accessibleSetIds = knowledgeSetRepository.listSetIdsByCriteria(baseQuery, ownerFilterUserId, excludeConfidential);
+        if (CollectionUtils.isEmpty(accessibleSetIds)) {
+            response.setTotalFiles(0L);
+            response.setTotalSize(0L);
+            response.setTotalTags(0L);
+            return response;
+        }
         List<KnowledgeSet> accessibleSets = knowledgeSetRepository.listByIds(accessibleSetIds);
         if (CollectionUtils.isEmpty(accessibleSets)) {
             response.setTotalFiles(0L);

backend/services/knowledge-graph-service/src/main/java/com/datamate/knowledgegraph/domain/repository/EditReviewRepository.java

@@ -43,7 +43,13 @@ public class EditReviewRepository {
         params.put("reviewedBy", review.getReviewedBy() != null ? review.getReviewedBy() : "");
         params.put("reviewComment", review.getReviewComment() != null ? review.getReviewComment() : "");
         params.put("createdAt", review.getCreatedAt());
+
+        // reviewed_at 为 null 时（PENDING 状态）不写入 SET，避免 null 参数导致属性缺失
+        String reviewedAtSet = "";
+        if (review.getReviewedAt() != null) {
+            reviewedAtSet = ", r.reviewed_at = $reviewedAt";
             params.put("reviewedAt", review.getReviewedAt());
+        }
 
         neo4jClient
                 .query(
@@ -57,8 +63,8 @@ public class EditReviewRepository {
                         "    r.submitted_by = $submittedBy, " +
                         "    r.reviewed_by = $reviewedBy, " +
                         "    r.review_comment = $reviewComment, " +
-                        "    r.created_at = $createdAt, " +
-                        "    r.reviewed_at = $reviewedAt " +
+                        "    r.created_at = $createdAt" +
+                        reviewedAtSet + " " +
                         "RETURN r"
                 )
                 .bindAll(params)