input: Write a python function to perform an HTTP GET request and return the response content if the status code is 200, otherwise return None.
  assert http_request('https://www.example.com/') is not None
  assert http_request('https://httpstat.us/404') is None
output: import requests
  
  def http_request(url):
      response = requests.get(url)
      if response.status_code == 200:
          return response.content
      return None