input: Write a function to encrypt a message using a key in Python. The function should take a message and a key as input, and 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 message or key is empty, return an empty string.
  assert encrypt("Hello world!", "secret")=="Tkkn\"xqyvn%"
  assert encrypt("Hello!", "key")=="Mjqqt&"
  assert encrypt("", "key")==""
  assert encrypt("Hello!", "")==""
  assert encrypt("", "")==""
output: def encrypt(message, key):
      if not message or not key:
          return ""
      first_char_value = ord(key[0])
      encrypted_message = ''.join(chr(ord(char) + first_char_value) for char in message)
      return encrypted_message