input: Write a function to count the number of words in a given string. The function should return the number of words in the string. If the string is empty, the function should return 0.
  assert count_words('Hello world, this is a sentence.')==5
  assert count_words('Python is fun')==3
  assert count_words('')==0
  assert count_words('One')==1
  assert count_words('This is a test with multiple words.')==7
output: def count_words(s):
      if not s:
          return 0
      return len(s.split())