input: Write a function to change the case of a given string. The function should provide methods to convert the string to upper case, lower case, and title case. If the string is None, it should return None for each case conversion method.
  
  class StringCaseModifier:
      def __init__(self, string):
          self.string = string
  
      def to_upper(self):
          return self.string.upper() if self.string else None
  
      def to_lower(self):
          return self.string.lower() if self.string else None
      
      def to_title(self):
          return self.string.title() if self.string else None
  
  if __name__ == '__main__':
      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(None).to_upper()==None
      assert StringCaseModifier(None).to_lower()==None
      assert StringCaseModifier(None).to_title()==None
output: class StringCaseModifier:
      def __init__(self, string):
          self.string = string
  
      def to_upper(self):
          return self.string.upper() if self.string else None
  
      def to_lower(self):
          return self.string.lower() if self.string else None
      
      def to_title(self):
          return self.string.title() if self.string else None