input: Write a function to print out Pascal's triangle for a given number of rows. The function should return None and print each row of the triangle within the function. If the given number of rows is less than 1, the function should print nothing and return None.
  assert pascal_triangle(5) is None
  assert pascal_triangle(0) is None
  assert pascal_triangle(-3) is None
output: def pascal_triangle(n):
      if n < 1:
          return None
      for i in range(n):
          row = [1] * (i + 1)
          for j in range(1, i):
              row[j] = prev_row[j-1] + prev_row[j]
          prev_row = row
          print(' '.join(map(str, row)))