If there are no H2s to populate the right-nav TOC, hide it

[sanscontext]

On this page:
http://segment-docs-stage.s3-website-us-west-2.amazonaws.com/docsv2/guides/best-practices/best-practices-identify/

There are no H2s, and so the right side-nav is confusingly empty. If this is the case, we should hide the "On this page" label.