Make sure <figcaption> element has text
Make sure <figcaption> element has text

Make sure an asset's ``<figcaption>`` element has text before
copying it for a popup caption. Some assets may have ``<figcaption>``
elements containing things like a list of datasets, or attribution,
but no caption text.

Addresses #717

denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase