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 an encrypted message. The encryption process should involve adding the ASCII value of each character in the message to the ASCII value of the first character in the key. If the message or key is empty, the function should return an empty string.
  
  assert encrypt("Hello world!", "secret")=="Tfqp'xsso$."
  assert encrypt("Python", "key")=="[^|vsr"
  assert encrypt("", "key")==""
  assert encrypt("message", "")==""
  assert encrypt("", "")==""
output: def encrypt(message, key):
      if not message or not key:
          return ""
      encrypted_message = ""
      key_char = ord(key[0])
      for char in message:
          encrypted_message += chr(ord(char) + key_char)
      return encrypted_message