GMs: fail gracefully for null surface

A couple more instances where MakeRenderTarget returns nullptr
due to --preAbandonGpuContext.

BUG=skia5282
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1976163002

Review-Url: https://codereview.chromium.org/1976163002
2 files changed