input: Write a function to count the number of words in a string. The function should return the number of words as an integer. If the input string is empty, the function should return 0.
  
  assert count_words('Hello world, this is a sentence.') == 5
  assert count_words('This is a test.') == 4
  assert count_words('') == 0
  assert count_words('One') == 1
  assert count_words('Two words') == 2
output: def count_words(s):
      if not s:
          return 0
      return len(s.split())