input: Write a function to create an application in Python for encryption and decryption of strings using the AES encryption algorithm. The function should take a plaintext string as input, encrypt it using a specified key, and return the encrypted string. Additionally, provide a function to decrypt the encrypted string back to its original form. If the plaintext or ciphertext is empty, return an empty string.
  
  from Crypto.Cipher import AES
  
  key = b'Sixteen byte key'
  
  def encrypt(plaintext):
      if not plaintext:
          return ""
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      if not ciphertext:
          return ""
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)
  
  assert encrypt("Hello World") != "Hello World"
  assert decrypt(encrypt("Hello World")) == "Hello World"
  assert encrypt("") == ""
  assert decrypt("") == ""
output: def encrypt(plaintext):
      if not plaintext:
          return ""
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      if not ciphertext:
          return ""
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)