PERF: Improve performance of StataReader's processing of missing values
Improve performance of StataReader when converting columns
with missing values

xref #25772

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