silenced two warnings in the docs

Author: SaturnFromTitan

doc/source/user_guide/missing_data.rst

@@ -190,15 +190,15 @@ The sum of an empty or all-NA Series or column of a DataFrame is 0.
 
    pd.Series([np.nan]).sum()
 
-   pd.Series([], dtype="float64").sum()
+   pd.Series([], dtype="float64", index=[]).sum()
 
 The product of an empty or all-NA Series or column of a DataFrame is 1.
 
 .. ipython:: python
 
    pd.Series([np.nan]).prod()
 
-   pd.Series([], dtype="float64").prod()
+   pd.Series([], dtype="float64", index=[]).prod()
 
 
 NA values in GroupBy