Doc build succeeds when it should fail

[bmorelli25]

Summary

There's an edge case bug where docs-builder should fail but does not. I'm not sure if this is a bug in docs-builder or in our GH Action.

Example

A writer added two frontmatter sections to a single file in elastic/docs-content#374:

---
mapped_urls:
  - https://www.elastic.co/guide/en/observability/current/apm-act-on-data.html
  - https://www.elastic.co/guide/en/serverless/current/observability-apm-act-on-data.html
---

---
navigation_title: "Act on data"
---

# Act on application data [apm-act-on-data]

The "build documentation" step logs a fail due to an ArgumentNullException:

:: info: DocumentationGenerator[0] Resolving tree
:: fail: Program[0] System.ArgumentNullException: Value cannot be null. (Parameter 'markdown')    at Markdig.Helpers.ThrowHelper.ArgumentNullException_markdown() + 0x27    at Markdig.Markdown.ToPlainText(String, TextWriter, MarkdownPipeline, MarkdownParserContext) + 0xa6    at 
...

However, after that very long error, the docs build exits successfully with:

:: info: Program[0] generate :: Finished in '00:00:00.2889647

This leads to a failure in the Upload Artifact step as there is nothing to upload in .artifacts/docs/html/.

Action logs

Here's the action logs: https://github.com/elastic/docs-content/actions/runs/13207072028/job/36873474178