input: Write a function to convert a given string to uppercase, lowercase, and title case. If the string is empty, return an empty string for all three conversions.
  class StringCaseModifier:
      def __init__(self, string):
          self.string = string
  
      def to_upper(self):
          return self.string.upper() if self.string else ""
  
      def to_lower(self):
          return self.string.lower() if self.string else ""
      
      def to_title(self):
          return self.string.title() if self.string else ""
  
  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 __init__(self, string):
      self.string = string
  
  def to_upper(self):
      return self.string.upper() if self.string else ""
  
  def to_lower(self):
      return self.string.lower() if self.string else ""
  
  def to_title(self):
      return self.string.title() if self.string else ""