Welcome to the forum.
The fact that there are no foreign key relationships on the relational model diagram probably means that no Foreign Keys have not been defined in the database.
There is a Discover Foreign Keys feature in Data Modeler. You can invoke this from the drop-down menu you get when you right-click over the node for the Relational Model in the Browser tree.
This offers potential foreign key relationships based on column-names, etc. This may help you find and create some of the FK relationships in Data Modeler.
Regarding your second question, Data Modeler does not provide any functionality to identify empty tables. You will need to use a SQL query for this.