code: def stringify_col(df, col_name):
      
      df = df.copy()
      df[col_name] = df[col_name].fillna("")
      df[col_name] = df[col_name].astype(str)
      return df
docs: Take a dataframe and string-i-fy a column of values.
      Turn nan/None into "" and all other values into strings.
  
      Parameters
      ----------
      df : dataframe
      col_name : string