Add support for Django's {% empty %} in for-loops

[mxmerz]

I am missing support for an {% empty %} statement in for-loops. I know the {% empty %} statement from Django's templating language:

The for tag can take an optional {% empty %} clause whose text is displayed if the given array is empty or could not be found:

<ul>
{% for athlete in athlete_list %}
    <li>{{ athlete.name }}</li>
{% empty %}
    <li>Sorry, no athletes in this list.</li>
{% endfor %}
</ul>

[Keats]

I think I would rather wrap the forloop in a if rather than add the empty statement to keep things simple

[mxmerz]

Wrapping the for-loop in an if is, of course, possible – but template authors would have to do this for practically every for-loop.

<ul>
{% if athlete_list is defined and athlete_list|length > 0 %}
	{% for athlete in athlete_list %}
		<li>{{ athlete.name }}</li>
	{% endfor %}
{% else %}
	<li>Sorry, no athletes in this list.</li>
{% endif %}
</ul>

For my taste, this is too much boilerplate code in the template.

[Keats]

I'll take a PR for it