input: Write a function to encrypt a message using a key in Python. The function should take two parameters: the message and the key. It should return the encrypted message by adding the ASCII value of each character in the message to the ASCII value of the first character of the key. If the key is an empty string, return the original message.
  assert encrypt("Hello world!", "secret")=="Tfqqi!{|sv%"
  assert encrypt("Hello", "key")=="Pmttw"
  assert encrypt("Test", "")=="Test"
  assert encrypt("", "secret")==""
  assert encrypt("", "")==""
output: def encrypt(message, key):
      if not key:
          return message
      key_char = ord(key[0])
      encrypted_message = ''.join(chr(ord(char) + key_char) for char in message)
      return encrypted_message