Should warn if the document is empty

The Glade Document hash for an empty document, ie:
```html

```
is `"0"`, this is intended, but because there is a chance glade just ran too early before the content inside is ready (for example in react) we should probably warn the user that their document is empty.

Maybe if we observe mutations on inner DOM and react to them this is a non-issue

No file or test is named. Start by locating the code that computes the Glade Document hash for the glade-annotatable element, then inspect how inner-DOM readiness is handled for React-rendered content. Done means an empty document produces a user warning without incorrectly warning when content is added later.

Written by the indexing model from the issue text.