Java - consulta si tabla sql esta vacía

public void Cargarnum(){

       PreparedStatement pst = null;

       ResultSet rs = null;

       ResultSet rst = null;

       String num = "1";

       String ncomp = "";

       int entero;

       int total;

       String total2 = "";

       try{

           String sql = "SELECT count(*) FROM ventas_clientes";

           pst =connect().prepareStatement(sql);

           rst = pst.executeQuery();

        }catch(Exception e){

            System.err.println("Error" + e);

        }

        if(rst == null){

            lblncomp.setText(num);

        }else{

            try{

                String sql = "SELECT ncomp FROM ventas_clientes";

                pst =connect().prepareStatement(sql);

                rs = pst.executeQuery();

                while(rs.next()){

                    ncomp = rs.getString("ncomp");

                    entero = Integer.parseInt(ncomp);

                    total = entero +1;

                    total2 = Integer.toString(total);

                }

                lblncomp.setText(total2);

            }catch(Exception e){

                System.err.println("Error" + e);

            }

        }

   }

public void Cargarnum() {

    PreparedStatement pst = null;

    ResultSet rs = null;

    ResultSet rst = null;

    Connection cn = null;

    String num = "1";

    String ncomp = "";

    int entero;

    int total;

    String total2 = "";

    try {

        String sql = "SELECT count(*) FROM ventas_clientes";

        cn = connect();

        pst = cn.prepareStatement(sql);

        rst = pst.executeQuery();

        while(rst.next()){

            lblncomp.setText(rst.getString(1));

        }

    } catch (Exception e) {

        System.err.println("Error" + e);

    } finally {

        try{

            if (cn != null && !cn.isClosed()) {

                cn.close();

            }

        }catch (SQLException ex) {

            ex.printStackTrace(System.out);

        }

    }

}