Uploading empty DataFrame setting clean=True & col_names=True writes two rows with headers

```
    d2g.upload(df,
               spreadsheet_key,
               wks_name,
               credentials=credentials,
               col_names=True,
               row_names=False,
               clean=True)
```
as per title, when uploading an empty DataFrame with clear=True, the sheet is correctly emptied but it still writes 2 rows of data corresponding to the DataFrame headers.