code: def fillna(s: TextSeries, replace_string="") -> TextSeries:
      """
      Replaces not assigned values with empty or given string.
  
      Examples
      --------
      >>> import texthero as hero
      >>> import pandas as pd
      >>> import numpy as np
      >>> s = pd.Series(["I'm", np.NaN, pd.NA, "You're"])
      >>> hero.fillna(s)
      0       I'm
      1
      2
      3    You're
      dtype: object
      >>> hero.fillna(s, "Missing")
      0        I'm
      1    Missing
      2    Missing
      3     You're
      dtype: object
      """
  
      return s.fillna(replace_string).astype("str")
  
imports: from gensim.models.phrases import Phrases
  import re
  import string
  from typing import Optional, Set
  import unicodedata
  import numpy
  import pandas
  from texthero._types import TokenSeries, TextSeries, InputSeries
  from typing import List, Callable, Union
  import warnings
label_desc: Use image node_id 1 for calling a global function with example usage: fillna(s, replace_string) and returns: s