Fix crash for closing shortcode with no .Inner set

Fixes #6857
Closes #7330

hugolib/shortcode_test.go

@@ -1316,3 +1316,23 @@ title: "Hugo Rocks!"
 	}
 
 }
+
+// https://github.com/gohugoio/hugo/issues/6857
+func TestShortcodeNoInner(t *testing.T) {
+	t.Parallel()
+
+	b := newTestSitesBuilder(t)
+
+	b.WithContent("page.md", `---
+title: "No Inner!"
+---
+{{< noinner >}}{{< /noinner >}}
+
+
+`).WithTemplatesAdded(
+		"layouts/shortcodes/noinner.html", `No inner here.`)
+
+	err := b.BuildE(BuildCfg{})
+	b.Assert(err.Error(), qt.Contains, `failed to extract shortcode: shortcode "noinner" has no .Inner, yet a closing tag was provided`)
+
+}