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
dataSetChooser = new Combo( composite, SWT.BORDER | SWT.READ_ONLY );
		dataSetChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		dataSetChooser.addSelectionListener( new SelectionAdapter( ) {
			public void widgetDefaultSelected( SelectionEvent e )
			{
				refreshColumns( false );
			}
		}
		);
		createDataSet = new Button( composite, SWT.PUSH );
		createDataSet.setText( BUTTON_CREATE_DATA_SET );
		setButtonLayoutData( createDataSet );
		createDataSet.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{
				new NewDataSetAction( ).run( );
				refreshDataSets( );
			}
		} );
		createLabel( composite, LABEL_SELECT_VALUE_COLUMN );
		
		columnChooser = new Combo( composite, SWT.BORDER | SWT.DROP_DOWN );
		columnChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		columnChooser.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{
				updateButtons( );
			}
		} );
		Button valueColumnExprButton = new Button( composite, SWT.PUSH );