Fill all null values with NaN values in a column.
        Null values are ``None`` or en empty string

        :param col: column name
        :type col: str

        :example: ``ds.fill_nulls("mycol")``

def fill_nulls(self, col: str):
        """
        Fill all null values with NaN values in a column.
        Null values are ``None`` or en empty string

        :param col: column name
        :type col: str

        :example: ``ds.fill_nulls("mycol")``
        """
        n = [None, ""]
        try:
            self.df[col] = self.df[col].replace(n, nan)
        except Exception as e:
            self.err(e)