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
+ "/List"; 
	private static final String PARAM_CONTROL_COMBO = DesignChoiceConstants.PARAM_CONTROL_LIST_BOX
			+ "/Combo"; 
	private static final String DISPLAY_NAME_CONTROL_LIST = Messages.getString( "ParameterDialog.DisplayLabel.List" ); 
	private static final String DISPLAY_NAME_CONTROL_COMBO = Messages.getString( "ParameterDialog.DisplayLabel.Combo" ); 
	private static final String NONE_DISPLAY_TEXT = Messages.getString( "ParameterDialog.Label.None" ); 
	private static final Image DEFAULT_ICON = ReportPlatformUIImages.getImage( IReportGraphicConstants.ICON_DEFAULT );
	private static final Image ERROR_ICON = ReportPlatformUIImages.getImage( ISharedImages.IMG_OBJS_ERROR_TSK );
	private static final String STANDARD_DATE_TIME_PATTERN = "MM/dd/yyyy hh:mm:ss a"; 
	private HashMap dirtyProperties = new HashMap( 5 );
	private ArrayList choiceList = new ArrayList( );
	private static final IChoiceSet DATA_TYPE_CHOICE_SET = DEUtil.getMetaDataDictionary( )
			.getElement( ReportDesignConstants.SCALAR_PARAMETER_ELEMENT )
			.getProperty( ScalarParameterHandle.DATA_TYPE_PROP )
			.getAllowedChoices( );
	private static final IChoiceSet CONTROL_TYPE_CHOICE_SET = DEUtil.getMetaDataDictionary( )
			.getChoiceSet( DesignChoiceConstants.CHOICE_PARAM_CONTROL );
	private ScalarParameterHandle inputParameter;
	private boolean loading = true;
	private Text nameEditor, promptTextEditor, helpTextEditor, formatField;
	
	private Label promptMessageLine;
	
	private CLabel errorMessageLine;
	
	private Button isRequired, doNotEcho, isHidden, needSort, distinct;
	
	private Button importValue, changeDefault, changeFormat, createDataSet;
	
	private Button dynamicRadio, staticRadio;