input: Write a function to count the number of words in a given string. If the string is empty, return 0.
  assert word_count('Hello world, this is a sentence.')==5
  assert word_count('This is a test.')==4
  assert word_count('')==0
  assert word_count('One.')==1
output: def word_count(s):
      if not s:
          return 0
      return len(s.split())