upgrader: make sure to return a result only if there is problems

bt5/erp5_upgrader/SkinTemplateItem/portal_skins/erp5_upgrader/TemplateTool_checkTableConsistency.xml

@@ -56,7 +56,11 @@ If `fixit` is True, then it will create/drop table or alter existing tables\n
 """\n
 portal = context.getPortalObject()\n
 show_source = not(fixit)\n
-return [portal.portal_catalog.upgradeSchema(src__=show_source)]\n
+sql_src = portal.portal_catalog.upgradeSchema(src__=show_source)\n
+result = []\n
+if sql_src:\n
+  result.append(sql_src)\n
+return result\n
 </string> </value>
         </item>
         <item>