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. If the message is empty, return an empty string.
  assert encrypt("Hello world!", "secret")=="Tqxx{,u{vn("
  assert encrypt("Python", "key")=="".join(chr(ord(char) + ord('k'[0])) for char in "Python")
  assert encrypt("", "anykey")==""
  assert encrypt("Special characters!@#$%^&*()", "spcl")=="".join(chr(ord(char) + ord('s'[0])) for char in "Special characters!@#$%^&*()")
  assert encrypt("Data Encryption Standard", "DES")=="".join(chr(ord(char) + ord('D'[0])) for char in "Data Encryption Standard")
output: def encrypt(message, key):
      if not message:
          return ""
      shift = ord(key[0])
      return ''.join(chr(ord(char) + shift) for char in message)