Don't index into SystemInfo gpus if it's empty.
Don't index into SystemInfo gpus if it's empty.

angle::GetSystemInfo may return success on some platforms without
writing any entires into the gpus vector. Make sure we don't index
into this vector when it's empty.

Bug:chromium:1166338
Change-Id: I4cafeadf7aa9ac55b25a1afa04c8d28a2edf52e0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2628140
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle