fix(theme): <Admonition> should render properly without heading/icon

[andrmaz]

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• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

closes #8568

Test Plan

Test links

https://deploy-preview-10080--docusaurus-2.netlify.app/tests/pages/markdown-tests-mdx/#admonitions

Related issues/PRs

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 58	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 58	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 74	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 70	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 63	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 71	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report

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[andrmaz]

I need to fix the markdown test. I'm on it