Support non existent variables

More or less all other Django template tags allow you to pass in a variable name for a variable not present in the template context. In all these cases, the variable is simply ignored.

This PR adds support for this convention.  We needed this to avoid lots of conditional checks (do x, y and z variables exist? If so, run phased with this, or this, or this) wrapped around our `{% phased .. %}` template tags.

Based on the test cases, this behavior appears intentional, but I believe it may be counter-intuitive.