sas7bdat: Check if the SAS file has zero variables
If the given SAS file has 0 rows, throw an error for the EmptyData file.
When reading, check that the column information is available. If not,
throw an error.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
