fix: handle None case for cell_value in xlsx_parse_table_handle

--bug=1064042 --user=刘瑞斌 【工作流知识库】QA问答对分段时，分段的问题为空，导入后关联问题为 None https://www.tapd.cn/62980211/s/1806084

apps/common/handle/impl/table/xlsx_parse_table_handle.py

@@ -40,7 +40,8 @@ class XlsxParseTableHandle(BaseParseTableHandle):
                         if cell.coordinate in merged_range:
                             cell_value = sheet[merged_range.min_row][merged_range.min_col - 1].value
                             break
-
+                if cell_value is None:
+                    cell_value = ''
                 image = image_dict.get(cell_value, None)
                 if image is not None:
                     cell_value = f'![](./oss/file/{image.id})'