input: Provide a Python function to compute and return the MD5 hash of a given string. If the input string is empty, the function should return None.
  assert generate_md5('Hello world') == '3e25960a79dbc69b674cd4ec67a72c62'
  assert generate_md5('') == None
  assert generate_md5('A B C') == '0ef78513b0cb8cef12743f5aeb35f888'
  assert generate_md5('password') == '5f4dcc3b5aa765d61d8327deb882cf99'
output:     
      import hashlib
  
      if s == "":
          return None
      return hashlib.md5(s.encode()).hexdigest()
  