resources.ToCSS produces "stdin" in sourcemap sources if no "@include" is present in SCSS file

[stlufi]

What version of Hugo are you using (hugo version)?

$ hugo version
Hugo Static Site Generator v0.80.0/extended linux/amd64 BuildDate: unknown

Does this issue reproduce with the latest release?

yes

Bug Description

When using Hugo pipe for SASS/SCSS with option "enableSourceMap" true in a theme's template the resulting sourcemap shows the following behavior:

• style.scss without @import

  body {
    margin: 0;
  }

  results in soucemap style.css.map

  {
    "version": 3,
    "file": "style.css",
    "sourceRoot": "/www/my-hugo-page",
    "sources": [
      "stdin"
    ],
    "sourcesContent": [
      "<the content of style.scss>"
    ],
    "names": [],
    "mappings": "<the mappings>"
  }

• style.scss with @import pointing to an empty file

  body {
    margin: 0;
  }
  
  @import "dummy/empty"

  results in soucemap style.css.map

  {
    "version": 3,
    "file": "style.css",
    "sourceRoot": "/www/my-hugo-page",
    "sources": [
      "themes/my-hugo-theme/assets/css/style.scss",
      "themes/my-hugo-theme/assets/css/dummy/_empty.scss"
    ],
    "sourcesContent": [
      "<the content of style.scss>",
      ""
    ],
    "names": [],
    "mappings": "<the mappings>"
  }

The resulting source map contains the expected value if the sources array contains more than one item, otherwise it simply contains stdin. This makes debugging CSS via browser tooling very hard, especially when using more than one SCSS style sheet on the same page.

The respective Hugo pipe was called like this in themes/my-hugo-theme/layout/_default/baseof.html:

{{ $style := resources.Get "css/style.scss" | resources.ToCSS (dict "enableSourceMap" true) }}
<link href="{{ $style.RelPermalink }}" rel="stylesheet" type="text/css"/>

I believe the issue is caused by this piece of code in the Hugo sources: resources/resource_transformers/tocss/scss/tocss.go line 156
To me it looks like it expects to find stdin",, but this is not the case for a SCSS file without an @import, because there is simply no , after the "stdin" in this case (there is only one item in the array and a trailing comma is not needed).

[github-actions]

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