input: Write a function that constructs a class in Python which implements two methods to add two numbers and three numbers respectively. The first method, `add_two_numbers`, should take two arguments and return their sum. The second method, `add`, should take three arguments and return their sum. If the given inputs are empty, the method should return None.
  
  class Calculator:
   def __init__(self):
     pass
      
   def add_two_numbers(self, a, b):
     return a + b
  
   def add(self, a, b, c):
     return a + b + c
  
  assert Calculator().add_two_numbers(1, 2) == 3
  assert Calculator().add(1, 2, 3) == 6
  assert Calculator().add_two_numbers(0, 0) == 0
  assert Calculator().add(0, 0, 0) == 0
  assert Calculator().add_two_numbers(-1, -1) == -2
  assert Calculator().add(-1, -1, -1) == -3
output: 
  class Calculator:
   def __init__(self):
     pass
      
   def add_two_numbers(self, a, b):
     if a is None or b is None:
       return None
     return a + b
  
   def add(self, a, b, c):
     if a is None or b is None or c is None:
       return None
     return a + b + c
  