BUG: setting pd.NA into Series casts to object
- [x] closes #44199
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Example
```
ser = pd.Series([0.0, 1.0])
ser[0] = pd.NA
>>> ser.dtype
dtype('O')   # <- main
dtype('float64')  # <- PR
```
Can you give a code example of what actually changed? (what is the dtype now?) It's a bit hard to directly see from the code changes what it does in practice > Can you give a code example of what actually changed? (what is the dtype now?) It's a bit hard to directly see from the code changes what it does in practice

updated OP with example lgtm