input: Write a function to create a web scraper that extracts specific data from a given website using the Python package BeautifulSoup. The function should accept the URL of the website as input, parse the webpage, find the required HTML elements using tags and attributes, and return the extracted data in the desired format.
  
  assert create_web_scraper("http://example.com")==["http://example.com/page1", "http://example.com/page2"]
  assert create_web_scraper("http://another-example.com")==["http://another-example.com/resource1", "http://another-example.com/resource2"]
  assert create_web_scraper("http://invalid-url.com")==None
  assert create_web_scraper("")==None
output: def create_web_scraper(url):
      if not url:
          return None
      try:
          import requests
          from bs4 import BeautifulSoup
          response = requests.get(url)
          if response.status_code != 200:
              return None
          soup = BeautifulSoup(response.content, 'html.parser')
          links = soup.find_all('a', href=True)
          return [link['href'] for link in links if url in link['href']]
      except:
          return None