input: Write a function to find the maximum element in a given array. The function should return the maximum element. If the given array is empty then return None.
  assert find_max_element([1, 10, 50, 3, 8])==50
  assert find_max_element([3, 5, 10, 1, 2])==10
  assert find_max_element([4, 3, 1, 2])==4
  assert find_max_element([3, 5, 4, 1, 2])==5
  assert find_max_element([])==None
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)