input: Write a function to create a class named StringCaseModifier that modifies the case of a given string. The class should have three methods: to_upper, to_lower, and to_title, each converting the string to upper case, lower case, and title case respectively. If no string is provided, the class should initialize with an empty string.
  
  class StringCaseModifier:
      def __init__(self, string):
          self.string = string if string is not None else ''
  
      def to_upper(self):
          return self.string.upper()
  
      def to_lower(self):
          return self.string.lower()
      
      def to_title(self):
          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() == ''
  assert StringCaseModifier(None).to_upper() == ''
  assert StringCaseModifier(None).to_lower() == ''
  assert StringCaseModifier(None).to_title() == ''
output: class StringCaseModifier:
      def __init__(self, string):
          self.string = string if string is not None else ''
  
      def to_upper(self):
          return self.string.upper()
  
      def to_lower(self):
          return self.string.lower()
      
      def to_title(self):
          return self.string.title()