Rework `ViewDataDictionary` to ensure `ModelMetadata` is never `null` (#1466)

This is the reanimation of #1453, now done against dev.  No significant changes from that PR after I addressed comments there.

See individual commits for details.

/cc @pranavkm @rynowak @yishaigalatzer 
- idea makes me nervous; could lead to hard-to-explain semantics around value types
- think we should work w/ this in place for a while and see if we hit any cases that don't Just Work(tm)