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([])==None
  assert find_max_element([-5, -10, -3, -8])==-3
  assert find_max_element([100, 200, 300, 400, 500])==500
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)