CREATE TABLE jtest (
  ID NUMBER(3) PRIMARY KEY,
  TEXT VARCHAR2(30) NULL
);
INSERT INTO jtest VALUES (1, 'Test1');
INSERT INTO jtest VALUES (2, 'NULL');

public class nullstring {

final static String JDBC_DRIVER = "oracle.jdbc.driver.OracleDriver";
final static String DB_URL      = "jdbc:oracle:thin:@myhost:1521:xe";
final static String DB_USER     = "myuser";
final static String DB_PASS     = "myuserpw";

public static void main(String[] args) throws ClassNotFoundException, SQLException, UnsupportedEncodingException {
    Class.forName(JDBC_DRIVER);

    Connection hDB = null;
    hDB = DriverManager.getConnection(DB_URL, DB_USER, DB_PASS);

    String sCelldata = "";
    Statement oStatement = null;
    List<Model> oDatalist = new ArrayList<>();

    oStatement = hDB.createStatement();
    ResultSet oResult = oStatement.executeQuery("SELECT id, text FROM jtest");

    while (oResult.next()) {
        ObservableList<String> aCells = FXCollections.observableArrayList();
        for (int i = 1 ; i < (oResult.getMetaData().getColumnCount() + 1); i++) {
            String test = "NULL";
            if ( test == "NULL" ) {
                System.out.println(">> 1111111111111111111111111111111");
            }
            if ( oResult.getString(i) == "NULL" ) {
                System.out.println(">> 2222222222222222222222222222222");
            }
            sCelldata = URLDecoder.decode(oResult.getString(i), "UTF-8");
            if ( sCelldata == "NULL" ) {
                System.out.println(">> 3333333333333333333333333333333");
            }

            aCells.add(sCelldata);
            System.out.println("> [" + i + "] " + sCelldata);
            sCelldata = "";
        }
        oDatalist.add(new Model(aCells));
    }

    hDB.close();
}
}

package nullstring;

import javafx.beans.property.SimpleStringProperty;
import javafx.collections.ObservableList;

public class Model {
    private final SimpleStringProperty id = new SimpleStringProperty("");
    private final SimpleStringProperty text = new SimpleStringProperty("");

    public Model() {

    }

    public Model(ObservableList<String> aParams) {
        setId(aParams.get(0));
        setText(aParams.get(1));
    }

    public SimpleStringProperty getId() {
        return id;
    }

    public void setId(String sId) {
        id.set(sId);
    }

    public SimpleStringProperty getText() {
        return text;
    }

    public void setText(String sText) {
        text.set(sText);
    }
}

>> 1111111111111111111111111111111
> [1] 1
>> 1111111111111111111111111111111
> [2] Test1
>> 1111111111111111111111111111111
> [1] 2
>> 1111111111111111111111111111111
> [2] NULL
>> 1111111111111111111111111111111