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/+/2628140Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>