input: Write a function to count the number of words in a given string. Return the count of words. If the given string is empty then return 0.
  assert count_words('Hello world, this is a sentence.')==5
  assert count_words('This is another example.')==4
  assert count_words('')==0
  assert count_words('Single')==1
  assert count_words('Two words')==2
output: def count_words(s):
      if not s:
          return 0
      return len(s.split())