When attempting to export a CSV file from the extension sometimes a file called blank.csv is exported, which has no contents.
When a CSV is imported into Magento it is copied into the var/export directory in addition to being uploaded to the database in the respective table. This is specifically for use with the export button so that the file that you import is the same as the file that is exported.
If the CSV is deleted or is otherwise inaccessible to the extension when attempting to use the export button the blank CSV, as described above, is presented instead.
The CSV Management article details other possible methods to export a CSV so that you can retrieve the shipping rules.