input: Write a function to count the number of words in a given string by splitting the string at each space and returning the length of the resulting list. If the string is empty, return 0.
  assert count_words("Hello world, this is a sentence.")==5
  assert count_words("This is another sentence.")==4
  assert count_words("")==0
  assert count_words("SingleWord")==1
  assert count_words("Two Words")==2
output: def count_words(s):
      if not s:
          return 0
      return len(s.split())