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
expressionBuilder.setExpressionProvier( new ParameterExpressionProvider( inputParameter,
						dataSetChooser.getText( ) ) );
				if ( expressionBuilder.open( ) == OK )
				{
					setExpression( displayTextChooser,
							expressionBuilder.getResult( ).trim( ) );
				}
			}
		} );
		createDefaultEditor( );
		
		Composite sorttingArea = new Composite( valueArea, SWT.NONE );
		GridData sorttingAreaGridData = new GridData( GridData.FILL_HORIZONTAL );
		sorttingAreaGridData.horizontalSpan = 2;
		sorttingArea.setLayoutData( sorttingAreaGridData );
		sorttingArea.setLayout( UIUtil.createGridLayoutWithoutMargin( 5, false ) );
		createLabel( sorttingArea, LABEL_SORT_KEY );
		sortKeyChooser = new Combo( sorttingArea, SWT.BORDER );
		sortKeyChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		sortKeyChooser.add( CHOICE_DISPLAY_TEXT );
		sortKeyChooser.add( CHOICE_VALUE_COLUMN );
		sortKeyChooser.setText( CHOICE_DISPLAY_TEXT );
		createLabel( sorttingArea, LABEL_SORT_DIRECTION );
		sortDirectionChooser = new Combo( sorttingArea, SWT.BORDER );
		sortDirectionChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		sortDirectionChooser.add( NONE_DISPLAY_TEXT );
		sortDirectionChooser.add( CHOICE_ASCENDING );
		sortDirectionChooser.add( CHOICE_DESCENDING );
		sortDirectionChooser.setText( LABEL_NULL );
		distinct = new Button( sorttingArea, SWT.CHECK );