input: Write a function to sort a given array in ascending order using bubble sort. If the input array is empty, return an empty array.
  
  assert bubble_sort([3, 7, 4, 1]) == [1, 3, 4, 7]
  assert bubble_sort([]) == []
output: def bubble_sort(arr):
      n = len(arr)
      for i in range(n):
          for j in range(0, n-i-1):
              if arr[j] > arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]
      return arr