input: Write a function to generate nested loops that print out the elements of three given lists in a new line for each combination of elements. The function should accept three lists and return None.
  assert nested_loop([1, 2, 3], [4, 5, 6], [7, 8, 9])==None
  # Output should be:
  # 1 4 7
  # 1 4 8
  # 1 4 9
  # 1 5 7
  # 1 5 8
  # 1 5 9
  # 1 6 7
  # 1 6 8
  # 1 6 9
  # 2 4 7
  # 2 4 8
  # 2 4 9
  # 2 5 7
  # 2 5 8
  # 2 5 9
  # 2 6 7
  # 2 6 8
  # 2 6 9
  # 3 4 7
  # 3 4 8
  # 3 4 9
  # 3 5 7
  # 3 5 8
  # 3 5 9
  # 3 6 7
  # 3 6 8
  # 3 6 9
  assert nested_loop([], [1], [2])==None
  # Output should be:
  # (empty output)
  assert nested_loop([1], [], [2])==None
  # Output should be:
  # (empty output)
  assert nested_loop([1], [2], [])==None
  # Output should be:
  # (empty output)
output: for i in lst1:
      for j in lst2:
          for k in lst3:
              print(i, j, k)