This PR supresses a warning when calculating the median of a pd.Series that is full of NAs. The warning is just letting you know that numpy got an empty array but correctly returns np.nan.

The warning is issued as follows:

1. When the series has NAs and you try to calculate the median, nanops.nanmedian is called.
2. When the array is full of NAs, the _mask in nanops.nanmedian.get_median is all False, so np.nanmedian(x[_mask]) gets an empty array.
3. np.nanmedian calls numpy.lib.nanfunctions._nanmedian which has a short-circuit path when the array is empty [ref].
4. This path calls np.nanmean. This creates an empty mask [ref] because the array is empty.
5. Then, the sum of the negated mask is used as the denominator of the mean, which causes the result to be np.nan (correct!).
6. The method issues a warning because the array was empty.

Another way to solve this would be to change get_median to explicitly return np.nan before the call to np.nanmedian, but that would involve tampering with the condition in the if, which has no comments so I'm unsure if changing it makes sense.

    def get_median(x, _mask=None):
        if _mask is None:
            _mask = notna(x)
        else:
            _mask = ~_mask
        all_na = _mask.all()
        if (not skipna and not all_na) or all_na:
            return np.nan
        with warnings.catch_warnings():
            # Suppress RuntimeWarning about All-NaN slice
            warnings.filterwarnings(
                        "ignore", "All-NaN slice encountered", RuntimeWarning
            )
            res = np.nanmedian(x[_mask])
        return res

which simplifies to if skipna: return np.nan I think.

I haven't added tests nor ensured all of the below passed not added comments because I want to ensure this is the preferred solution. Otherwise, I can edit get_median. Let me know!

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.