hugo: fix edge case causing duplicate h1 (#17959)

Some documents are missing a `title` front matter,
and instead use an H1 from the document body.
This change introduces a fix to render an external
h1 only if the front matter field is set.

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

layouts/_default/list.html

@@ -5,7 +5,9 @@
 {{ define "main" }}
   {{ partial "breadcrumbs-legacy.html" . }}
   <article class="prose max-w-none dark:prose-invert">
-    <h1>{{ .Title }}</h1>
+    {{ with .Title }}
+    <h1>{{ . }}</h1>
+    {{ end }}
     {{ .Content }}
   </article>
 {{ end }}

layouts/_default/single.html

@@ -5,7 +5,9 @@
 {{ define "main" }}
   {{ partial "breadcrumbs-legacy.html" . }}
   <article class="prose max-w-none dark:prose-invert">
-    <h1>{{ .Title }}</h1>
+    {{ with .Title }}
+    <h1>{{ . }}</h1>
+    {{ end }}
     {{ .Content }}
   </article>
 {{ end }}