input: Write a function to count the number of words in a given 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("Python is fun")==3
  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())