fix: improve problem list population in custom split strategy

--bug=1064029 --user=刘瑞斌 【知识库】工作流知识库使用高级分段时，开启文档标题或名称关联问题开关，分段失败 https://www.tapd.cn/62980211/s/1805605

Author: liuruibin

apps/application/flow/step_node/document_split_node/impl/base_document_split_node.py

@@ -147,10 +147,10 @@ def _generate_problem_list(
             if document_name_relate_problem and document_name:
                 problem_list.append(document_name)
         elif split_strategy == 'custom':
-            if paragraph_title_relate_problem:
-                problem_list.extend(paragraph_title_relate_problem)
-            if document_name_relate_problem:
-                problem_list.extend(document_name_relate_problem)
+            if paragraph_title_relate_problem and paragraph.get('title'):
+                problem_list.extend(paragraph.get('title'))
+            if document_name_relate_problem and document_name:
+                problem_list.extend(document_name)
         elif split_strategy == 'qa':
             if document_name_relate_problem and document_name:
                 problem_list.append(document_name)