title: how to bind the component to data bean vaadin on button click only
body: <p>consider a registration form with some fields as described in this bean </p>
  
  <pre><code> public class RegistrationEntity {
       private Long accountNumber;
       private String firstName;
       private String middleName;
       private String lastName;
       private Long phoneNumber;
       private String emailId;
       private Long communicationHouseNo;
       private String communicationStreet;
       private Long communicationPincode;
       private Long communicationLandLine;
       private String communicationLandmark;
       private Long permenantHouseNo;
       private String permenantStreet;
       private Long permenantPincode;
       private Long permenantLandLine;
       private String permenantLandmark;
   }
  </code></pre>
  
  <p>and the components as designed by this </p>
  
  <pre><code> RegistrationEntity bean = new RegistrationEntity();
   BeanItem&lt;RegistrationEntity&gt; item = new BeanItem&lt;RegistrationEntity&gt;(bean);
  
   fieldGroup =   new FieldGroup(item);
  
   firstName = new TextField("First Name");
   firstName.setRequired(true);
   firstName.setWidth(COMMON_FIELD_WIDTH);
   fieldGroup.bind(firstName, "firstName");
  
  
   lastName = new TextField("Last Name");
   lastName.setRequired(true);
   lastName.setWidth(COMMON_FIELD_WIDTH);
   fieldGroup.bind(lastName, "lastName");
  
   dateOfBirth = new DateField("Date of birth");
   dateOfBirth.setRequired(true);
   dateOfBirth.setWidth(COMMON_FIELD_WIDTH);
   fieldGroup.bind(dateOfBirth, "dateOfBirth");
  
   mothersMaidenName = new TextField("Mother's Maiden name;");
   mothersMaidenName.setRequired(true);
   mothersMaidenName.setWidth(COMMON_FIELD_WIDTH);
   fieldGroup.bind(mothersMaidenName, "motherName");
  
   email = new TextField("User:");
   email.setRequired(true);
   email.setInputPrompt("Your username (eg. joe@email.com)");
   email.addValidator(new EmailValidator(
              "Username must be an email address"));
   email.setInvalidAllowed(false);
   email.setWidth(COMMON_FIELD_WIDTH);
   fieldGroup.bind(email, "emailId");
  
   panNo = new TextField("PAN:");
   panNo.setRequired(true);
   panNo.setWidth(COMMON_FIELD_WIDTH);
   fieldGroup.bind(panNo, "panNumber");
  </code></pre>
  
  <p>this binds the components with null values on initialization<br>
  but i want the components to display empty values instead of null<br>
  and when user clicks a particular button then only the values should be binded to the bean which can be later used for persistence </p>