input: Write a function to create a Python class that modifies the case of a given string. The class should have methods to convert the string to uppercase, lowercase, and title case. If the input string is empty, the methods should return an empty string.
  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: class StringCaseModifier:
      def __init__(self, input_string):
          self.input_string = input_string
  
      def to_upper(self):
          return self.input_string.upper() if self.input_string else ""
  
      def to_lower(self):
          return self.input_string.lower() if self.input_string else ""
  
      def to_title(self):
          return self.input_string.title() if self.input_string else ""