Don't allow judge to submit empty sub-questions

[matiroy]

trafficstars

Reported by Liam through Typeform on 8/26/2019 3:01:45.

Ex.: https://mosaic.ought.org/w/6773?e=efcded60-7f33-41f3-9d88-8e1ae677e017

[matiroy]

also reported by Matthias on Aug 28th (probably both referring to the same instance)

discussion reference: https://app.slack.com/client/T3L8K3NMT/CL6PM7L94/thread/CL6PM7L94-1567025045.017400?thread_ts=1564795608.023600