Add dummy object for validation mode.

[mattcoley]

If we are in validation mode and we have a zero-size loop, we used to convert [] to [0] so we can validate the for loop body by resolving it. The issue is as so looking at some typical code:

{% for test in get_objects() %}
 {{ test.property }}
 {{ test.numProperty + 10 }}
{% endfor %}

If get_objects() returns an empty list, we force test to be 0 which will cause validation errors in the for loop body where it expects a different object type returned from get_objects().

The best solution I can think of is to return a DummyObject instead of 0. The dummy object will resolve all properties (so they don't throw errors in cases like {{ test.property.evenDeeperProperty }}) and can be coerced to any type (so they don't throw errors in cases like {{ test.numProperty + 100 }}).