BUG: Fixed exception when Series.str.get is used with dict values and the index is not an existing key (#20671)
- [X] closes #20671
- [X] tests added / passed (* unrelated tests failed, with locale problems, not sure if caused by something on my system or something on pandas)
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry
