input: Write a function to create an application in Python for encryption and decryption of strings. The application should take a string as input, and use the AES cipher algorithm in ECB mode to encrypt and decrypt the string. If the given string is empty then return an empty string.
  assert encrypt("Hello, World!")!=""
  assert decrypt(encrypt("Hello, World!"))=="Hello, World!"
  assert encrypt("")==""
  assert decrypt("")==""
output: def encrypt(plaintext):
      if not plaintext:
          return ""
      from Crypto.Cipher import AES
      from Crypto.Util.Padding import pad
      key = b'Sixteen byte key'
      cipher = AES.new(key, AES.MODE_ECB)
      ciphertext = cipher.encrypt(pad(plaintext.encode(), AES.block_size))
      return ciphertext.hex()
  
  def decrypt(ciphertext):
      if not ciphertext:
          return ""
      from Crypto.Cipher import AES
      from Crypto.Util.Padding import unpad
      key = b'Sixteen byte key'
      cipher = AES.new(key, AES.MODE_ECB)
      plaintext = unpad(cipher.decrypt(bytes.fromhex(ciphertext)), AES.block_size)
      return plaintext.decode()