input: Write a function to print the following output pattern for a given integer input n. If n is 0 or negative, the function should print "Input must be a positive integer." The function header should be `def print_pattern(n):`.
  assert print_pattern(5)==None
  assert print_pattern(0)=="Input must be a positive integer."
  assert print_pattern(-3)=="Input must be a positive integer."
  assert print_pattern(3)==None
output: def print_pattern(n):
      if n <= 0:
          print("Input must be a positive integer.")
          return
      for i in range(1, n + 1):
          print('*' * i)