input: Write a function to find the maximum element in the given array. If the given array is empty then return None.
  assert find_max_element([1, 10, 50, 3, 8])==50
  assert find_max_element([-1, -10, -50, -3, -8])==-1
  assert find_max_element([7])==7
  assert find_max_element([10, 20, 30, 40, 50])==50
  assert find_max_element([])==None
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)