Walk through using empty template tag the first time

[treyhunner]

We the {% empty %} template tag twice throughout the tutorial and we leave it as an exercise to the reader to figure it out both times.

First time in: http://test-driven-django-development.readthedocs.org/en/latest/03-views.html#updating-our-views

Second time in: http://test-driven-django-development.readthedocs.org/en/latest/07-templatetags.html#making-it-a-bit-more-robust

I think we should walk through how to use it one of these times (probably the first?).