Short description of error
rendering a cycles render with an empty strokes set gives a disproportionately long render time (producing no strokes) and will make the background of the render black.
Exact steps for others to reproduce the error
open a default scene
move the cube out of view
use the cycles render engine
set the background to transparent (Render > Film > transparent)
render an image
what you should notice is a "strokes set empty" message in the console. when debugging is enabled, you'll also note that the stroke render time (Stroke rendering) is quite high, even though the system should already know that there are no strokes to be rendered.
I believe that the Freestyle render process could and should be exited as soon as an empty strokes set is detected. it makes no sense to continue. Also, as far as I tested, this is a cycles-only problem (both the background and the long render time).
This problem was reported to me by @Thomas Beck (plasmasolutions)