input: Write a function to define a Python class named StringCaseModifier that can modify the case of a given string. The class should have methods to convert the string to uppercase, lowercase, and title case. If the string is empty, it should 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: 
  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()
  