(a)

CREATE TABLE WidgetWithName (widget_id INT NOT NULL PRIMARY KEY /* REFERENCES Widget ?? */);

CREATE TABLE WidgetName (widget_id INT NOT NULL REFERENCES WidgetWithName (widget_id), widget_principal_name_id INT NOT NULL);

Using a null in place of a value that exists but is unknown would be a less accurate representation because null in SQL does not accurately represent the unknown case. In my example I can accurately represent a widget with a name attribute, a widget without and a widget with a name which is not known.

(b) No table with a null is normalized because all the classic normal forms are defined only for relations without nulls (see E.F.Codd quoted earlier in this thread). To have a sound basis for saying that a table with a null is normalized you would therefore have to reinvent those definitions, in which case I think you ought to state clearly what definitions you are going to use. For example what is a join dependency on a table with nulls? In SQL "nulls don't join" therefore a SQL table with a null is not the join of the projections on its superkeys and therefore it is not in Fifth Normal Form according to the conventional definition of a join dependency.