[v10.2.x] Templating: Json interpolation of single-value default selection does not create valid json

[grafana-delivery-bot]

Backport 31d79c0 from #79137

---

Fixes an issue where, if a template variable with options doesn't have a value selected, the default value picked wasn't wrapped in an array as it would be if a user were to select it themselves.

Effectively the old value was

{
    value: option.value,
    text: option.text,
    selected: true,
}

where the new value is

{
    value: typeof option.value === 'string' ? [option.value] : option.value,
    text: typeof option.text === 'string' ? [option.text] : option.text,
    selected: true,
}

So value and text are wrapped in an array if they are plain strings.