input: Write a function to implement an application in Python for encryption and decryption of strings using the AES algorithm. The function should use a 16-byte key and ECB mode for encryption and decryption. If the input or output string is not applicable, the function should return None.
  from Crypto.Cipher import AES
  
  key = b'Sixteen byte key'
  
  def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)
  
  assert encrypt(b'hello world 1234') == b'P\xb1m\xb2%\x1a\xc45\x10BD"\xc9 \xc0\xae\x1c'
  assert decrypt(b'P\xb1m\xb2%\x1a\xc45\x10BD"\xc9 \xc0\xae\x1c') == b'hello world 1234'
  assert encrypt(None) == None
  assert decrypt(None) == None
output: def encrypt(plaintext):
      if plaintext is None:
          return None
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      if ciphertext is None:
          return None
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)