BUG: pd.to_numeric does not copy _mask for ExtensionArrays 
- [x] closes #38974 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry (not needed as bug on master only)

Fixing a bug I introduced in  #38746
thanks @arw2019 