DefaultEntityResolver: use the HTML5 entities as fallback if no subset is found in getExternalSubset()

Currently, `DefaultEntityResolver.getExternalSubset()` retrieves external subsets for the html and svg element names and returns `null` otherwise. However, it could be modified to return the HTML5 entities DTD as a fallback, instead of `null`.

That would allow documents with no `DOCTYPE` and where the root element is neither `html` nor `svg` to still be able to use entities.