input: Write a function to count the number of words in a given string. Split the string by spaces and return the total count of resulting words. If the input string is empty, return 0.
  assert count_words('Hello world, this is a sentence.')==5
  assert count_words('')==0
  assert count_words('Single')==1
  assert count_words('   Space separated   ')==2
  assert count_words('Multiple     spaces')==2
output: def count_words(s):
      if not s:
          return 0
      return len(s.split())