The aircraft was cruising at flight level 170 in clear conditions at night. Although no thunderstorms had been forecast, the crew observed some electrical activity on the intended track. The weather radar was reported to have been functioning normally and detected scattered cells. The pilot under check elected to divert to the right of the track to remain clear of those cells and also activated the seatbelt warning chimes twice to alert the nursing sister seated in the cabin. The Captain reported that, although it was dark, ground lights and the weather radar indicated that there were no cloud build-ups ahead of the aircraft after the diversion. Shortly after, the aircraft encountered an area of severe turbulence associated with wispy cloud. During that encounter, one of two stretcher patients sustained minor facial injuries after striking nearby equipment. Both stretcher patients had been restrained at the time of the occurrence.