This is not obvious, but in order to make 1: 1 to 1: 0-1, you must change the “required” checkbox in the relationship editor.
First click on the 1: 1 tool, then click on the column in the table that you want to see on the "0" side, and then the column on the "1" side. Now double-click the connector line and in the editor window, under the tab "foreign key", uncheck the box "required" on the "0" side of the relationship. The symbol in the diagram must be changed accordingly.
Hope this helps.