Bug 153523 OK button in parameter dialog is enabled without default value in combo-box
Description: OK button in parameter dialog is enabled without default value in combo-box Steps to reproduce: 1. New a report parameter, combo-box. 2. Click New button, input abc as value, click OK 3. Set abc as default value and close Edit Parameter dialog. 4. New a data source and dataset 5. Edit the parameter, remove abc value and import values from dataset Expected result: OK button is disabled for there is no default value. Actual result: OK button is enabled.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
*            the title of the dialog
	 */
	public ParameterDialog( String title )
	{
		this( UIUtil.getDefaultShell( ), title );
	}
	/**
	 * Create a new parameter dialog with given title under the specified shell
	 * 
	 * @param parentShell
	 *            the parent shell of the dialog
	 * @param title
	 *            the title of the dialog
	 */
	public ParameterDialog( Shell parentShell, String title )
	{
		super( parentShell, title );
	}
	protected Control createDialogArea( Composite parent )
	{
		Composite composite = (Composite) super.createDialogArea( parent );
		createPropertiesSection( composite );
		createMoreOptionSection( composite );
		UIUtil.bindHelp( parent, IHelpContextIds.PARAMETER_DIALOG_ID );
		return composite;
	}
	private void createPropertiesSection( Composite composite )
	{
		Group propertiesSection = new Group( composite, SWT.NONE );
		propertiesSection.setText( GROUP_PROPERTIES );