BUG: isna not returning copy for MaskedArray
- [x] closes #40935
- [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

Think further improvements like zero-copy access fall under #34873, so #40935 can be closed.
thanks @mzeitlin11 