Don't show sector metadata if there is none

The `sector_tag_finder` always returns true because it is an object that
exists. We need to also test that there are sectors which want
displaying before adding the data to the metadata block.

https://www.pivotaltracker.com/story/show/68445322
Merge pull request #1391 from alphagov/optionally-show-sectors

Don't show sector metadata if there is none

https://www.pivotaltracker.com/story/show/68445322