BUGFIX - AttributeError raised in StataReader.value_labels()
- [x] closes #19417 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Returns empty dict instead of raising AttributeError for very old Stata files. Includes test against very old Stata file.