LPS-40267 Export model count is 0 for portlets using the DefaultConfigurationPortletDataHandler.

A return value of -1 indicates that the portletDataHandler may export some content, but it does not implement the export model count logic. Portlets using the DefaultConfigurationPortletDataHandler or any of its subclasses only export portlet preferences, so the export model count is 0.
LPS-40267 Remove unnecessary null checking for portletDataHandlerClass
LPS-40267 Fix condition to show the export/import icon.

This fixes LPS-40267 consistently with the logic in ExportImportHelperImpl and preserves LPS-37542.