Transcriptions without zones are not supported in elements search

Sentry Issue: ARKINDEX-FRONTEND-DS

TypeError: e is null
  at u (./js/helpers/iiif.js:47:19)
  at call (./vue/Image/ZoneWithContext.vue?9971:1:207)
...
(42 additional frame(s) were not displayed)

Transcriptions without zones cause a TypeError when the ZoneWithContext component tries to get an IIIF URL for a null polygon. The search API needs an update to add a zone attribute on the returned elements as well as a prefetch_related('zone__image__server') on elements so that the frontend can still display something; once backend#310 (closed) is completed, zones on transcriptions can be removed entirely.