BUG: AssertionError on Series.append(DataFrame) fix  #30975 
- [x] closes #30975 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Series.append() does not throw AssertionError anymore. This fix performs a precheck before proceeding with concatenation. Tests have passed. Whatsnew entry added.
Thanks!