Inclusion tags should handle arguments not present in the context gracefully

[kuon]

If we call an inclusion tag like that

{% my_tag object %}

and object is not present in the context, an exception will be raised, thus obliging us to do:

{% if object %}
{% my_tag object %}
{% else %}
{% my_tag %}
{% endif %}

It would be better to have the inclusion tag mechanism to handle keys not present in the context and just default them to None.

[kuon]

Duplicate of #12283 , sorry about that.