Handle undefined body and html for rte

What does this MR do and why?

Handles undefined body and html for rte

When the body and html from the render markdown response are undefined, fall back to an empty string

In the case that the markdown text is \n\n, the /preview_markdown response will have an empty string as the body and a missing html value. Previously, this would result in the body becoming undefined (as body from the response is falsy, causing a short-circuit to the undefined html value).

This change makes the case described above fall back to an empty string, which would be the expected outcome.

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Before	After
Screen_Recording_2025-03-11_at_10.15.42	Screen_Recording_2025-03-11_at_10.16.15

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Relates to #523885 (closed)