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
distinct.setText( CHECKBOX_DISTINCT );
		addCheckBoxListener( distinct, CHECKBOX_DISTINCT );
		createLabel( valueArea, null );
		createPromptLine( valueArea );
		listLimit.setEditable( true );
	}
	private void clearDefaultValueText( )
	{
		if ( defaultValueChooser == null )
			return;
		String textValue = defaultValueChooser.getText( );
		if ( textValue != null
				&& textValue.equals( CHOICE_NULL_VALUE )
				|| textValue.equals( CHOICE_BLANK_VALUE ) )
		{
			defaultValueChooser.setText( "" );
		}
	}
	private void clearDefaultValueChooserSelections( )
	{
		if ( defaultValueChooser == null )
			return;
		if ( defaultValueChooser.getItemCount( ) > 1 )
		{
			defaultValueChooser.remove( CHOICE_NULL_VALUE );
			defaultValueChooser.remove( CHOICE_BLANK_VALUE );
		}
	}
	private void createDefaultEditor( )
	{