If we pass no bytes, use the fuzz binary itself.

This is mostly for convenient local testing.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1587043009

Review URL: https://codereview.chromium.org/1587043009
1 file changed