input: Write a function to create a web scraper that utilizes BeautifulSoup to extract specific data from a given website. The function should parse the webpage, find the required HTML elements using tags and attributes, and return the extracted data in the desired format.
  assert web_scraper("<url_of_webpage>")==['<url_1>', '<url_2>']
  assert web_scraper("<another_url>")==['<data_item_1>', '<data_item_2>']
  assert web_scraper("<yet_another_url>")==['<extracted_info>']
  assert web_scraper("")==None
output: def web_scraper(url):
      if not url:
          return None
      import requests
      from bs4 import BeautifulSoup
      response = requests.get(url)
      soup = BeautifulSoup(response.text, 'html.parser')
      elements = soup.find_all('a', href=True)
      return [elem['href'] for elem in elements]