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 as an integer.
  assert count_words("Hello world, this is a sentence.")==5
  assert count_words("This is a test.")==4
  assert count_words("One.")==1
  assert count_words("")==0
  assert count_words("Multiple words here!")==3
output: def count_words(s):
      if not s:
          return 0
      return len(s.split())