input: Write a function to find the maximum element in a given array. If the array is empty, return None.
  assert find_max_element([1, 10, 50, 3, 8]) == 50
  assert find_max_element([5, 3, 2, 7, 1]) == 7
  assert find_max_element([-1, -10, -50, -3, -8]) == -1
  assert find_max_element([]) == None
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)