Introduce the concept of nil field

Still learning my way around tasmo so I may not have everything covered.

This should allow nil field to be used in empty sub-view levels to cause the model path hash to
not collide.  The field is added on every field write for model paths which include nil and is removed
upon object deletion.  This does incur extra write cost but I'm not seeing an alternative (yet?) and
I've isolated it to the (hopefully) rare case.