input: Write a function to find the largest value from 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([3, 7, 1, 5])==7
  assert get_max_value([-2, -5, -1])==-1
  assert get_max_value([])==None
output: def get_max_value(numbers):
      if not numbers:
          return None
      return max(numbers)