input: Write a function to capitalize the first character of each word in a given string. The function should take a string as input and return a new string with each word 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("python programming")== "Python Programming"
  assert capitalize_string("hello world")== "Hello World"
  assert capitalize_string("")== ""
  assert capitalize_string("a b c d e")== "A B C D E"
output: def capitalize_string(s):
      if not s:
          return ""
      return ' '.join(word.capitalize() for word in s.split())