Avoid lxml fail if text is blank string when applying markup filter

If not:
* the API doesn't work for all those languages, projects,... with empty
descriptions,
* it fails when accessing those fields in the shell.

Steps to replicate the bug:

* Open the shell:

        lang = Language.objects.get(code="eu")
        lang.description.rendered # Fails. It is the same error as in the API.

* Add a description for the language (In Pootle) and reload the shell:

        lang = Language.objects.get(code="eu")
        lang.description.rendered # Works.