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/CascadingParametersDialog.java
{
		String displayFormat;
		IChoiceSet choiceSet = getFormatChoiceSet( getSelectedDataType( ) );
		if ( choiceSet == null )
		{
			displayFormat = DEUtil.getMetaDataDictionary( )
					.getChoiceSet( DesignChoiceConstants.CHOICE_STRING_FORMAT_TYPE )
					.findChoice( DesignChoiceConstants.STRING_FORMAT_TYPE_UNFORMATTED )
					.getDisplayName( );
		}
		else
		{
			displayFormat = choiceSet.findChoice( formatCategroy )
					.getDisplayName( );
			if ( isCustom( ) )
			{
				displayFormat += ":  " + formatPattern; 
			}
		}
		formatField.setText( "" + displayFormat ); 
		changeFormat.setEnabled( choiceSet != null );
		if ( selectedParameter != null )
		{
			doPreview( isCustom( ) ? formatPattern : formatCategroy );
		}
	}
	private boolean isCustom( )
	{
		if ( DesignChoiceConstants.STRING_FORMAT_TYPE_CUSTOM.equals( formatCategroy )
				|| DesignChoiceConstants.NUMBER_FORMAT_TYPE_CUSTOM.equals( formatCategroy )