Make GrGLGetDriverInfo work for null strings.

The media unit tests in Chromium use a test context that returns nullptr for its vendor/renderer strings.

TBR=robertphillips@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1831633002

Review URL: https://codereview.chromium.org/1831633002
1 file changed