Fix crawling error in AsyncWebCrawler

[theguy000]

Related to #105

Fix the 'NoneType' object has no attribute 'get' error in AsyncWebCrawler.

• crawl4ai/async_webcrawler.py

  • Add a check in the arun method to ensure html is not None before further processing.
  • Raise a descriptive error if html is None.

• crawl4ai/async_crawler_strategy.py

  • Add a check in the crawl method of the AsyncPlaywrightCrawlerStrategy class to handle cases where html is None.
  • Raise a descriptive error if html is None.

• tests/async/test_basic_crawling.py

  • Add a test case to verify handling of None values for the html variable in the test_invalid_url function.

• tests/async/test_error_handling.py

  • Add a test case to verify handling of None values for the html variable in the test_network_error function.

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