Text handler is not invoked for empty content

I would expect that any element with empty content and a closing tag to fire the `text` handler with `text: ''` and `lastInTextNode: true`. Otherwise, I do not get a chance to append content (based on existing content). Alternatively, an endtag handler mentioned in https://github.com/cloudflare/lol-html/issues/85 would suffice, if it gives me a chance to use `endTag.before()`.

The issue names no files or tests. Start by locating the parser path for empty content and the text or end-tag handler API, then add coverage for an empty element with a closing tag. Done means the callback receives empty text with lastInTextNode true, or the end-tag hook provides the requested before() capability.

Written by the indexing model from the issue text.