Fix clang report failure when no warnings.

Clang report fails when there are no warnings to report.
Report directory is empty (and it gets erased), so `cp` command after `scan-build` fails.

@justinmk I haven't been able to test this, so it may need some minor adjustment. But the idea should work.
Gosh. Can't believe how much is this going to resist!
I need learning how to work locally with bot-related things, to be able to test them.
Send you new PR in a minute.