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
final SelectionChoice choice = (SelectionChoice) element;
			boolean isDefault = isDefaultChoice( choice );
			SelectionChoiceDialog dialog = new SelectionChoiceDialog( Messages.getString( "ParameterDialog.SelectionDialog.Edit" ) ); 
			dialog.setInput( choice );
			dialog.setValidator( new SelectionChoiceDialog.ISelectionChoiceValidator( ) {
				public String validate( String displayLabel, String value )
				{
					return validateChoice( choice, displayLabel, value );
				}
			} );
			if ( dialog.open( ) == Dialog.OK )
			{
				choice.setValue( convertToStandardFormat( choice.getValue( ) ) );
				if ( isDefault )
				{
					changeDefaultValue( choice.getValue( ) );
				}
				return true;
			}
			return false;
		}
		public boolean newItem( )
		{
			SelectionChoice choice = StructureFactory.createSelectionChoice( );
			SelectionChoiceDialog dialog = new SelectionChoiceDialog( Messages.getString( "ParameterDialog.SelectionDialog.New" ) ); 
			dialog.setInput( choice );
			dialog.setValidator( new SelectionChoiceDialog.ISelectionChoiceValidator( ) {
				public String validate( String displayLabel, String value )
				{
					return validateChoice( null, displayLabel, value );