input: Write a function to find a pair of numbers from a given list that add up to a specified target. If such a pair exists, return the pair as a list. Otherwise, return None. If the input list is empty, return None.
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 8)==[3, 5]
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 9)==[3, 6]
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 11)==None
  assert find_pair([], 8)==None
output: def find_pair(nums, target):
      seen = set()
      for num in nums:
          complement = target - num
          if complement in seen:
              return [complement, num]
          seen.add(num)
      return None