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, 5, 5])==5
  assert find_max_element([-1, -2, -3, -4])==-1
  assert find_max_element([1])==1
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)