input: Write a function to encrypt a message using a key in Python. The function should take a message and a key as inputs and return the encrypted message by adding the ASCII value of the first character of the key to each character in the message. If the key is empty, return the original message.
  assert encrypt("Hello world!", "secret")=="Tqxxa iadry!"
  assert encrypt("Python", "key")=="[syviq"
  assert encrypt("Encryption", "z")=="Fodsbqwfsj"
  assert encrypt("Test", "")=="Test"
  assert encrypt("", "any")==""
output: def encrypt(message, key):
      if not key:
          return message
      key_value = ord(key[0])
      encrypted_message = ''.join(chr(ord(char) + key_value) for char in message)
      return encrypted_message