input: Write a function to find the maximum element in the given array. If the array is empty, return None.
  assert find_max_element([1, 10, 50, 3, 8])==50
  assert find_max_element([100, 0, -1, 50])==100
  assert find_max_element([9])==9
  assert find_max_element([])==None
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)