input: Write a function to capitalize the first character of each word in a given string. The function should return the modified string with each word's first character capitalized. If the input string is empty, the function should return an empty string.
  assert capitalize_string("this is an example") == "This Is An Example"
  assert capitalize_string("hello world") == "Hello World"
  assert capitalize_string("capitalize each word") == "Capitalize Each Word"
  assert capitalize_string("a b c d e") == "A B C D E"
  assert capitalize_string("") == ""
output: def capitalize_string(s):
      if not s:
          return ""
      return ' '.join(word.capitalize() for word in s.split())