admonitions() breaks when an admonition just before "^---\n"

[dreampuf]

Thank you for the great tool. I found a corner case of content that would break markdown2's rendering.

https://github.com/tencentcloudstack/terraform-provider-tencentcloud/blob/master/website/docs/r/cdn_domain.html.markdown

---
subcategory: "Content Delivery Network(CDN)"
layout: "tencentcloud"
page_title: "TencentCloud: tencentcloud_cdn_domain"
sidebar_current: "docs-tencentcloud-resource-cdn_domain"
description: |-
  Provides a resource to create a CDN domain.
~> **NOTE:** To disable most of configuration with switch, just modify switch argument to off instead of remove the whole block\n <--- This new line sign will be stripped in admonitions()
---

And later, markdown.convert will fail to extra metadata.

Traceback (most recent call last):
  File "/opt/./scripts/render.py", line 426, in <module>
    main()
  File "/opt/./scripts/render.py", line 80, in main
    body = render_markdown(text=input_data, flavor=args.flavor)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/./scripts/render.py", line 129, in render_markdown
    md = markdown2.markdown(text=text, extras=extras)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/.venv/lib/python3.11/site-packages/markdown2.py", line 185, in markdown
    use_file_vars=use_file_vars, cli=cli).convert(text)
                                          ^^^^^^^^^^^^^
  File "/opt/.venv/lib/python3.11/site-packages/markdown2.py", line 356, in convert
    text = self._extract_metadata(text)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/.venv/lib/python3.11/site-packages/markdown2.py", line 488, in _extract_metadata
    tail = fence_splits[2]
           ~~~~~~~~~~~~^^^
IndexError: list index out of range