input: Write a function to determine the largest value in a given list of numbers. If the list is empty, return None.
  assert get_max_value([2, 4, 1, 6])==6
  assert get_max_value([10, 5, 20, 30])==30
  assert get_max_value([-1, -5, -2, -3])==-1
  assert get_max_value([1])==1
  assert get_max_value([])==None
output: def get_max_value(numbers):
      if not numbers:
          return None
      return max(numbers)