### What problem does this PR solve?

fix "File contains no valid workbook part"

stacktrace:
```
Traceback (most recent call last):
  File "/ragflow/deepdoc/parser/excel_parser.py", line 54, in _load_excel_to_workbook
    return RAGFlowExcelParser._dataframe_to_workbook(df)
  File "/ragflow/deepdoc/parser/excel_parser.py", line 69, in _dataframe_to_workbook
    ws.cell(row=row_num, column=col_num, value=value)
  File "/ragflow/.venv/lib/python3.10/site-packages/openpyxl/worksheet/worksheet.py", line 246, in cell
    cell.value = value
  File "/ragflow/.venv/lib/python3.10/site-packages/openpyxl/cell/cell.py", line 218, in value
    self._bind_value(value)
  File "/ragflow/.venv/lib/python3.10/site-packages/openpyxl/cell/cell.py", line 197, in _bind_value
    value = self.check_string(value)
  File "/ragflow/.venv/lib/python3.10/site-packages/openpyxl/cell/cell.py", line 165, in check_string
    raise IllegalCharacterError(f"{value} cannot be used in worksheets.")
```

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):