<img src> fails assert

[mooffie]

The following:

<img src alt="popup-image">

...raises an AssertionError because src exists in the element's attributes yet it's None.

The "problematic" source code:

    if tag == "img" and start and not self.ignore_images:
        if "src" in attrs:
            assert attrs["src"] is not None

• html2text version: 2020.1.16
• Python version: 3.11.4