BUG: Series.combine() fails with ExtensionArray inside of Series
- [x] closes #20825
closes #21248
- [x] tests added / passed
   - extension/category/test_categorical.py:test_combine
   - extension/decimal/test_decimal.py:test_combine
   - series/test_combine_concat.py:test_combine_scalar
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
   - modified `whatsnew.v0.24.0.txt`

This is a split of #20889 to just fix the `combine` issue.


