AssertionError for img src attribute

Some web pages have errors. Rather than simply throwing an exception, it would be better to ignore benign errors and convert as much of the page as possible.

- Version by `html2text --version`: 2020.1.16
- Python version `python --version`: Python 3.7.7

- Test script:
```
import requests
import html2text

rsp = requests.get('https://blog.logrocket.com/from-rest-to-graphql/')
h2t = html2text.HTML2Text()
h2t.ignore_links = True
h2t.bypass_tables = False
text = h2t.handle(rsp.text)
```

- Log:
```
File "c:\users\arvindpdmn\miniconda3\lib\site-packages\html2text\__init__.py", line 142, in handle
self.feed(data)
File "c:\users\arvindpdmn\miniconda3\lib\site-packages\html2text\__init__.py", line 139, in feed
super().feed(data)
File "c:\users\arvindpdmn\miniconda3\lib\html\parser.py", line 111, in feed
self.goahead(0)
File "c:\users\arvindpdmn\miniconda3\lib\html\parser.py", line 171, in goahead
k = self.parse_starttag(i)
File "c:\users\arvindpdmn\miniconda3\lib\html\parser.py", line 345, in parse_starttag
self.handle_starttag(tag, attrs)
File "c:\users\arvindpdmn\miniconda3\lib\site-packages\html2text\__init__.py", line 191, in handle_starttag
self.handle_tag(tag, dict(attrs), start=True)
File "c:\users\arvindpdmn\miniconda3\lib\site-packages\html2text\__init__.py", line 502, in handle_tag
assert attrs["src"] is not None
AssertionError
```