parser/pageparser: Fix when only shortcode and then summary

Fixes #5464

hugolib/page_test.go

@@ -1508,6 +1508,15 @@ title: "Hugo"
 This is a {{< sc >}}.
 <!--more--> 
 Content.
+`)
+
+	// https://github.com/gohugoio/hugo/issues/5464
+	b.WithContent("page-md-only-shortcode.md", `---
+title: "Hugo"
+---
+{{< sc >}}
+<!--more--> 
+{{< sc >}}
 `)
 
 	b.WithContent("page-md-shortcode-same-line.md", `---
@@ -1570,6 +1579,11 @@ CONTENT:{{ .Content }}
 		"SUMMARY:<p>Summary.</p>:END",
 		"CONTENT:<p>Summary.</p>\n\n<p>Content.\t</p>\n",
 	)
+
+	b.AssertFileContent("public/page-md-only-shortcode/index.html",
+		"SUMMARY:a shortcode:END",
+		"CONTENT:a shortcode\n\na shortcode\n",
+	)
 }
 
 // TODO(bep) this may be useful for other tests.