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.
I think we can avoid all this boilerplate with @fwalch template approach: https://github.com/neovim/bot-ci/commit/e32a348f1bcfa38efa2c5a8646583c00d3c7cc40#diff-668f09e02f4b078544a35b39156b004dR36

I will make these changes when I get time if you don't want to.