Return error when .Render is invoked without arg

Fixes #11243

hugolib/page__per_output.go

@@ -557,6 +557,9 @@ func (p *pageContentOutput) RenderWithTemplateInfo(ctx context.Context, info tpl
 }
 
 func (p *pageContentOutput) Render(ctx context.Context, layout ...string) (template.HTML, error) {
+	if len(layout) == 0 {
+		return "", errors.New("no layout given")
+	}
 	templ, found, err := p.p.resolveTemplate(layout...)
 	if err != nil {
 		return "", p.p.wrapError(err)

hugolib/page_test.go

@@ -1983,3 +1983,25 @@ title: "p2"
 	b.Assert(identity.HashString(p1), qt.Not(qt.Equals), identity.HashString(p2))
 	b.Assert(identity.HashString(sites[0]), qt.Not(qt.Equals), identity.HashString(sites[1]))
 }
+
+// Issue #11243
+func TestRenderWithoutArgument(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/index.html --
+{{ .Render }}
+`
+
+	b, err := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			Running:     true,
+		},
+	).BuildE()
+
+	b.Assert(err, qt.IsNotNil)
+
+}