input: Write a python function to find the maximum element in a given array.
  assert maximum_element([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 10
  assert maximum_element([-10, -20, -3, -4, -5]) == -3
  assert maximum_element([]) == None
output: def maximum_element(arr):
      if not arr:
          return None
      return max(arr)