Fix: Slippers tag without children mistaken for opening tag

[jnns]

Slippers template tags can be used with and without children:

{% photo src='file.png' %}

vs.

{% #photo %}
  <img src="file.png">
{% /photo %}

In case both versions are used alongside, {% photo %} will be wrongly classified as an opening tag because {% /photo %} follows later in the file.