Re list bug fix

Fixes oja-list-bug #184

Also changes the _amd.html_ example not to use custom elements, as this caused issues when testing this (_NSERROR_ when doing `insertOrderedList` with the caret at the end of a 1 line paragraph).
Also not sure about using `textContent === ''` here, for the same reason.

This time, the P element might be empty, or it might contain a BR element.

[We insert BR elements into empty Ps](https://github.com/guardian/scribe/blob/master/src/plugins/core/formatters/html/ensure-selectable-containers.js). So if the P was empty to begin with, it probably will have a BR (we don't bother cleaning these up, [yet](https://github.com/guardian/scribe/issues/75)). If it wasn't empty to begin with, it won't have a BR. But we need to check for both cases.

Perhaps: `listParentNode.childNodes.length === 0 || (listParentNodes.childNodes.length === 1 && listParentNodes.childNodes[0].nodeName === 'BR')`.