input: Write a function that prints out the Pascal's triangle for a given number of rows. The function should take an integer `n` as input, which represents the number of rows to print in the triangle. If `n` is less than 1, the function should not print anything and return `None`. Otherwise, it should print the Pascal's triangle up to the `n`th row.
  
  
  def print_pascals_triangle(n): 
      if n < 1:
          return None
      trow = [1] 
      y = [0] 
      for x in range(n): 
          print(trow) 
          trow = [l + r for l, r in zip(trow + y, y + trow)] 
  
  assert print_pascals_triangle(5) == None
  assert print_pascals_triangle(0) == None
  assert print_pascals_triangle(1) == None
  
output: def print_pascals_triangle(n): 
      if n < 1:
          return None
      trow = [1] 
      y = [0] 
      for x in range(n): 
          print(trow) 
          trow = [l + r for l, r in zip(trow + y, y + trow)]