Only apply cache result progress if resulted in diff

Created by: eseliger

A user reported this, we store execution step results in the cache also when no diff was generated (rightfully), but then we should only apply the patch when there is actually anything to do.

Also, I initially couldn't reproduce this locally because we swallowed this error in volume mode. It would only happen in bind mode.

Closes https://github.com/sourcegraph/src-cli/issues/755

Test plan

Verified this does properly fail in volume mode now, too and don't fail anymore at all.