input: Write a python function to capitalize the first letter of a string.
  assert capitalize_first_letter("hello") == "Hello"
  assert capitalize_first_letter("") == ""
  assert capitalize_first_letter("world") == "World"
  assert capitalize_first_letter("python") == "Python"
  assert capitalize_first_letter("a") == "A"
output: def capitalize_first_letter(s):
      if s:
          return s[0].upper() + s[1:]
      return s