Make get_loc with nan for FloatIndex consistent with other index types
- [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

Have not added a whatsnew because I am not sure if 2 is expected here. You could also make a point for retruning nan. But to handle these cases consistently, we have to check for method here