Fix JSON tables with non-string values

Asherah Connorrequested to merge
push-koklrvuqrqrt into master

What does this MR do and why?

Fix JSON tables with non-string values.

Ensure strings are UTF-8 encoded when we pass them to GLFMMarkdown; it's not just the empty string we have to worry about, it turns out.

References

Fixes `json:table` with integer cell values raises `T... (#596559 - closed).

Screenshots or screen recordings

With this input:

```json:table
{"fields":[{"key":"a"}],
 "items":[{"a":42}],
 "markdown":true}
```
Before After
image image

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  2. Check this branch out.
  3. Repeat. Observe it succeeds.
  4. Observe.

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