input: Write a function to concatenate two strings "Hello" and "World" and print them on the same line. If the input strings are empty, print them directly without any concatenation.
  assert store_and_print("Hello", "World")=="Hello World"
  assert store_and_print("", "")==""
  assert store_and_print("Hello", "")=="Hello "
  assert store_and_print("", "World")==" World"
output: def store_and_print(str1, str2):
      if str1 == "" and str2 == "":
          return ""
      elif str1 == "":
          return " " + str2
      elif str2 == "":
          return str1 + " "
      else:
          return str1 + " " + str2