input: Write a function to print out all combinations of elements from three given lists in a new line using nested loops. If any of the given lists is empty, the function should not print anything.
  assert nested_loops([1,2,3], [4,5,6], [7,8,9])
  assert nested_loops([], [4,5,6], [7,8,9])==None
  assert nested_loops([1,2,3], [], [7,8,9])==None
  assert nested_loops([1,2,3], [4,5,6], [])==None
  assert nested_loops([], [], [])==None
output: def nested_loops(list1, list2, list3):
      if not list1 or not list2 or not list3:
          return
      for i in list1:
          for j in list2:
              for k in list3:
                  print(i, j, k)