input: Write a function to find the maximum element in a given array of numbers. If the array is empty, return None.
  assert find_max_element([1, 43, 78, 3, 92, 42])==92
  assert find_max_element([])==None
  assert find_max_element([34, 56, 23, 78, 102])==102
  assert find_max_element([10])==10
  assert find_max_element([-10, -20, -30, -5])==-5
output: def find_max_element(arr):
      if not arr:
          return None
      return max(arr)