erp5_ci_slideshow: support unobtainable creation dates (= None)

bt5/erp5_ci_slideshow/SkinTemplateItem/portal_skins/erp5_ci_slideshow/WebPage_viewAsWeb.py

@@ -87,7 +87,8 @@ document_content = removeEmptyDetails(document.getTextContent())
 document_theme = getThemeFromFirstFollowUpProduct(document_reference)
 document_title = document.getTitle()
 document_description = document.getDescription()
-document_creation_year = document.getCreationDate().strftime('%Y')
+document_creation_date = doc.getCreationDate()
+document_creation_year = document_creation_date.strftime('%Y') if document_creation_date else ''
 document_theme_logo_url = "NXD-Media.Logo." + document_theme.capitalize()
 document_theme_logo = context.restrictedTraverse(document_theme_logo_url)
 document_claim = document_theme_logo.getDescription()