Fix counter defaults on new model instances.

On new models, counters were being initialized to 0, since the model didn't
have a primary key set yet. This meant that we skipped any sort of calculations,
resulting in counters simply being broken.

Now we refuse to set a default, keeping it NULL, if it's a new model. The next
load should then perform a proper calculation.

Tested with an accompanying Review Board change. Test passes.