fix: markdown rendering panic when code blocks do not have languages (#12325)

When attempting to render a markdown code block that does not have a language set in it, Forgejo will fail to render and log an error:
```
2026/04/29 08:47:47 ...markdown/markdown.go:162:func1() [W] Unable to render markdown due to panic in goldmark: runtime error: invalid memory address or nil pointer dereference
```

This is a regression introduced by #12056.

## Checklist

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### Tests for Go changes

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [x] `make pr-go` before pushing

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
    - pre-release regression

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12325
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

modules/markup/markdown/markdown_test.go

@@ -1511,4 +1511,12 @@ func TestCodeblockLanguageStripping(t *testing.T) {
 			"```",
 		`<pre class="code-block"><code class="chroma language-rust display"><span class="k">fn</span> <span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{}</span><span class="w">
 </span></code></pre>`)
+
+	// No language identifier
+	test(
+		"```\n"+
+			"fn main() {}\n"+
+			"```",
+		`<pre class="code-block"><code class="chroma language-text display">fn main() {}
+</code></pre>`)
 }

modules/markup/markdown/transform_codeblock_lang.go

@@ -11,6 +11,9 @@ import (
 )
 
 func (g *ASTTransformer) transformCodeblockLanguage(v *ast.FencedCodeBlock, reader text.Reader) {
+	if v.Info == nil {
+		return
+	}
 	src := reader.Source()
 	info := v.Info.Segment.Value(src)
 	// Strip language after commas