The spec for public void JoinRowSet.addRowSet(Joinable rowset) says:
...
Throws:
    java.sql.SQLException - if (1) an empty rowset is added to the to this JoinRowSet object,
...

In fact, java.sql.SQLException is thrown if rowset is empty. See an example below:
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import com.sun.rowset.CachedRowSetImpl;
import com.sun.rowset.JoinRowSetImpl;
import java.sql.SQLException;
import java.sql.Types;
import javax.sql.rowset.CachedRowSet;
import javax.sql.rowset.JoinRowSet;
import javax.sql.rowset.RowSetMetaDataImpl;

/**
 *
 * @author aag
 */
public class NewClass {
    
    /** Creates a new instance of NewClass */
    public static void main(String[] argv) throws SQLException {
        JoinRowSet jrs = new JoinRowSetImpl();
        CachedRowSet crs = new CachedRowSetImpl();
        RowSetMetaDataImpl rsmd = new RowSetMetaDataImpl();
        rsmd.setColumnCount(1);
        rsmd.setColumnName(1, "FIELD1");
        rsmd.setColumnType(1, Types.INTEGER);
        crs.setMetaData(rsmd);
        try {
            jrs.addRowSet(crs, 1);
            System.out.println("SQLException was not thrown");
        } catch (SQLException e) {
            System.out.println("SQLException was thrown");
        }
    }
    
    
}
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run-single:
SQLException was not thrown