fix: inclusion of null locs in sitemap.xml

Impacted by Hugo changes

Fix #2732

wowchemy/layouts/_default/sitemap.xml

@@ -2,7 +2,7 @@
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
         xmlns:xhtml="http://www.w3.org/1999/xhtml">
   {{/* Check that page is built (has a URL) and is not private. */}}
-  {{ range where (where .Data.Pages "Permalink" "!=" nil) "Params.private" "ne" true }}
+  {{ range where (where .Data.Pages "Permalink" "!=" "") "Params.private" "ne" true }}
   <url>
     <loc>{{ .Permalink }}</loc>
     {{- if not .Lastmod.IsZero }}<lastmod>{{ safeHTML ( .Lastmod.Format "2006-01-02T15:04:05-07:00" ) }}</lastmod>{{ end }}