input: Write a function to store the strings "Hello" and "World" in two variables and print them in the same line. If the given input is empty, return an empty string.
  assert print_hello_world()=="Hello World"
  assert print_hello_world([])==""
output: def print_hello_world(input=None):
      if input is None:
          return "Hello World"
      else:
          return ""