RelationCounterField (and parts of CounterField) were testing

instances for truthiness to see if they were valid (i.e., references

haven't dropped), but if an instance defined __nonzero__ or __len__

(with a result of 0), the checks would fail and state would be lost.
These checks have changed to explicitly check against None, ensuring

that we don't accidentally lose anything in these cases, and to add

validity checks in places where they were missing.
While here, I also got rid of a method that was defined that was never

used. It was also assuming validity, and rather than fix it, it was

worth just getting rid of the function.

Unit tests pass. They failed with the addition of a __nonzero__

function in the models prior to these fixes.