Hide URL preview if it will be empty

[turt2live]

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After:

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A question with this is whether it's reasonable to not render the preview (implying the server might be slow/failing to the user), or if we want to put in some placeholder values instead.

Tests for this are difficult because URL previews. Ideally we'd use an integration test for this, but testing localhost/reliability of the preview is questionable.

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Here's what your changelog entry will look like:

✨ Features

• Hide URL preview if it will be empty (#9029).