code: def fill_nulls(self, col: str):
          
          n = [None, ""]
          try:
              self.df[col] = self.df[col].replace(n, nan)
          except Exception as e:
              self.err(e)
docs: 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")``