Bug 182673 No default value error message prompts incorrectly
Steps to reproduce: 1. New a report parameter; 2. Choose control type as Combo; 3. Choose parameter as dynamic type; Expected result: As the isRequired is false by default, parameter's default value can be null value. There should be no error message about no default value prompt. Actual result; Error messages prompts
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
changeFormat.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{
				popupFormatBuilder( true );
			}
		} );
		createLabel( displayOptionSection, null );
		Group previewArea = new Group( displayOptionSection, SWT.NONE );
		previewArea.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		previewArea.setLayout( UIUtil.createGridLayoutWithoutMargin( ) );
		previewArea.setText( LABEL_PREVIEW );
		previewLabel = new Label( previewArea, SWT.NONE );
		previewLabel.setAlignment( SWT.CENTER );
		previewLabel.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		
		createLabel( displayOptionSection, LABEL_LIST_LIMIT ); 
		Composite limitArea = new Composite( displayOptionSection, SWT.NULL );
		GridLayout layout = new GridLayout( 2, false );
		layout.marginWidth = 0;
		layout.marginHeight = 0;
		limitArea.setLayout( layout );
		GridData data = new GridData( GridData.FILL_HORIZONTAL );
		data.verticalSpan = 1;
		limitArea.setLayoutData( data );
		listLimit = new Text( limitArea, SWT.BORDER );
		data = new GridData( );
		data.widthHint = 80;
		listLimit.setLayoutData( data );
		listLimit.addVerifyListener( new VerifyListener( ) {
			public void verifyText( VerifyEvent e )