input: Write a function to modify the case of the string. The function should provide methods to convert the string to uppercase, lowercase, and title case. If the string is not provided, return an empty string for each case modification method.
  
  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: 
  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 ''
  