input: Write a function to find and return the maximum element in a 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([4, 3, 1, 2])==4
  assert find_max_element([])==None
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)