Test Case: pd.DataFrame.prob with min_count changes dtype if result contains NaNs  
- [ ] closes #47091 
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Hi @jreback

This PR is about to add test cases for pd.prob -  https://github.com/pandas-dev/pandas/pull/47091#issuecomment-1140514302 thanks @weikhor very nice!