In Gazebo8 Calls to DepthCamera()->ImageData() (DepthCameraSensor::ImageData) from a return an image with values that hover around 128.
Identical configuration and code using Gazebo7 returned a valid image which is the same as it would be with a normal CameraSensor.
Duplicating the camera in the sdf (one as type "camera" and one as type "depth") results in functional camera which works as a workaround to the apparently broken ImageData interface of the DepthCameraSensor.
I checked the history of rendering/DepthCamera.cc and rendering/Camera.cc and didn't see any smoking guns. sensors/DepthCameraSensor.cc and sensors/CameraSensor.cc were harder
Bug can be exercised by adding "<save enabled=1><path>./images</path></save>" below the "camera" tag of any currently working sdf sensor type="depth". Saved images will all be gray.