input: Write a function to automate the login process in a given website using Selenium in Python. Initialize the web driver, enter the provided username and password, click the submit button, and print "Login successful!" if the login is successful. If the given website is not specified, return "Invalid target website."
  
  assert login_automation('http://www.example.com') == 'Login successful!'
  assert login_automation('') == 'Invalid target website.'
output: def login_automation(website):
      if not website:
          return "Invalid target website."
      
      from selenium import webdriver
      from selenium.webdriver.common.by import By
      from selenium.webdriver.common.keys import Keys
  
      driver = webdriver.Chrome()
      driver.get(website)
      
      username = driver.find_element(By.ID, "username")
      password = driver.find_element(By.ID, "password")
      submit_button = driver.find_element(By.ID, "submit")
      
      username.send_keys("your_username")
      password.send_keys("your_password")
      submit_button.click()
      
      return "Login successful!"