Bug: SSR of malformed? `meta` renders nothing(working in 18.2)

[eps1lon]

React version:

Steps To Reproduce

1. SSR <meta name="test" /> (renderToString and renderToReadableStream)

Link to code example:

• Working in 18.2: https://codesandbox.io/s/18-2-ssr-meta-only-working-t6lxvw
• render nothing in 18.3-5fcf1a4b4c2150a1b9fe0de0144a82a053c63966: https://codesandbox.io/s/18-3-5fcf1a4b4c2150a1b9fe0de0144a82a053c63966-ssr-meta-only-bug-woguy0?file=/src/index.js

The current behavior

Renders empty string

The expected behavior

Renders <meta name="test" />
Don't know if that's a valid use case but it should at least warn that the meta tag is malformed?

This broke the attribute fixture (originally reported in #25927).

Also:

1. <meta name="description" content={content} /> renders nothing unless content is a string.
2. <meta http-equiv /> i.e. any http-equiv usage no longer renders anything. It used to warn at least.