Hg remove empty ps

This fixed the problem of <b> tags wrapping <p> tags and creating invalid HTML when pasting from Google Docs. 
> In documentation and code, refer to nodes by their canonical node name in uppercase. E.g. `P`.

– https://github.com/guardian/scribe/blob/master/CONTRIBUTING.md#conventions

We have to be disciplined because working on `contenteditable` is already enough of a mess!