input: Write a python function to create a class that manages a string, with methods to set, get, print, and get the length of the string.
  assert my_str.get_string() == ""
  my_str.set_string("Hello")
  assert my_str.get_string() == "Hello"
  assert my_str.get_length() == 5
  my_str.set_string("Hello, world!")
  assert my_str.get_length() == 13
output: class StringManager:
      def __init__(self):
          self._string = ""
      
      def set_string(self, string):
          self._string = string
      
      def get_string(self):
          return self._string
      
      def print_string(self):
          print(self._string)
      
      def get_length(self):
          return len(self._string)
  
  # Example usage:
  my_str = StringManager()
  assert my_str.get_string() == ""
  my_str.set_string("Hello")
  assert my_str.get_string() == "Hello"
  assert my_str.get_length() == 5
  my_str.set_string("Hello, world!")
  assert my_str.get_length() == 13