Fix 404 with base template regression

Fixes #6795

hugolib/404_test.go

@@ -30,3 +30,19 @@ func Test404(t *testing.T) {
 	b.AssertFileContent("public/404.html", "Not Found")
 
 }
+
+func Test404WithBase(t *testing.T) {
+	t.Parallel()
+
+	b := newTestSitesBuilder(t)
+	b.WithSimpleConfigFile().WithTemplatesAdded("404.html", `{{ define "main" }}
+Page not found
+{{ end }}`)
+	b.Build(BuildCfg{})
+
+	// Note: We currently have only 1 404 page. One might think that we should have
+	// multiple, to follow the Custom Output scheme, but I don't see how that would work
+	// right now.
+	b.AssertFileContent("public/404.html", `Page not found`)
+
+}