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 
Should we do a reference equality check here?

`if (modelType == source.ModelMetadata.ModelType && object.ReferenceEquals(model, source.ModelMetadata.Model))`