input: Write a class to modify the case of a string. Implement methods to convert the string to uppercase, lowercase, and title case. If the input string is empty, return an empty string for all methods.
  
  class StringCaseModifier:
      def __init__(self, string):
          self.string = string
  
      def to_upper(self):
          if self.string == "":
              return ""
          return self.string.upper()
  
      def to_lower(self):
          if self.string == "":
              return ""
          return self.string.lower()
      
      def to_title(self):
          if self.string == "":
              return ""
          return self.string.title()
  
  assert StringCaseModifier("Hello World").to_upper() == "HELLO WORLD"
  assert StringCaseModifier("Hello World").to_lower() == "hello world"
  assert StringCaseModifier("Hello World").to_title() == "Hello World"
  assert StringCaseModifier("").to_upper() == ""
  assert StringCaseModifier("").to_lower() == ""
  assert StringCaseModifier("").to_title() == ""
output: def to_upper(self):
      if self.string == "":
          return ""
      return self.string.upper()
  
  def to_lower(self):
      if self.string == "":
          return ""
      return self.string.lower()
  
  def to_title(self):
      if self.string == "":
          return ""
      return self.string.title()