input: Write a function to execute a valid insertion sort on a given list of integers. If the given list is empty then return the list as it is.
  
  def insertion_sort(arr):
      if not arr:
          return arr
      for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr
  
  assert insertion_sort([64, 34, 25, 12, 22, 11, 90]) == [11, 12, 22, 25, 34, 64, 90]
  assert insertion_sort([5, 3, 8, 4, 6]) == [3, 4, 5, 6, 8]
  assert insertion_sort([]) == []
  assert insertion_sort([1, 2, 3, 4, 5]) == [1, 2, 3, 4, 5]
  assert insertion_sort([5, 4, 3, 2, 1]) == [1, 2, 3, 4, 5]
output: def insertion_sort(arr):
      if not arr:
          return arr
      for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr