Light/dark toggle shown without dark stylesheet when the dark brand file holds only comments #15131

Since #15101, a brand file that holds only comments is ignored with a warning instead of stopping the render (#14891). When this is the dark file of a light/dark brand, the HTML page still gets the light/dark color-scheme toggle, but no dark stylesheet is generated, so the toggle does nothing.

doc.qmd:

---
title: test
brand:
  light: light.yml
  dark: dark.yml
---

light.yml:

color:
  primary: "#ff0000"

dark.yml holds only comments. Same result with the brand: block in _quarto.yml.

With brand: {light: light.yml} (no dark key) there is no toggle, which is the expected output when the dark file is ignored.

The toggle is decided from the dark key in the brand metadata, not from the brand that was loaded:

This is also used by website navigation, dashboards, giscus and brand Sass defaults, so the fix needs to keep explicit dark themes (theme: {light, dark}) working.