Language tabs break if highlight shortcodes are not adjacent.

If adjacent `\`< highlight >`` shortcodes have an extra return between them, it breaks the language switcher tabs. This appears to be happening because the Markdown parser generates empty `` tags for the return. This doesn't happen in all circumstances. It seems to happen if the first shortcode is preceded by a text element and _no_ return. The jQuery in language-switch-v2.js doesn't expect the `` tags, and the CSS probably doesn't either.

It _may_ be possible to fix this by removing `` tags. We shouldn't touch the Hugo markdown parser or builtin shortcodes.

Imported from Jira [BEAM-12688](https://issues.apache.org/jira/browse/BEAM-12688). Original Jira may contain additional context.
Reported by: pcoet.