dj-ref with empty content

[AbcSxyZ]

Conversion of variable reference will fail if the tag body is empty, e.g. <h1 dj-ref="blabla"></h1>.

Like https://github.com/lingster/django-bootstrap-studio-tools/issues/11, string will be None, so nothing will append.

If I add a space inside h1, it works.

Perhaps not so important if BSS always prettify his html.