Require diff comfirmation only when there is someting to confirm.

If for some reason the webkitpy tools can't create a pretty diff
then we should not ask if the diff was correct.

BUG=

Review URL: https://codereview.chromium.org/544593003

git-svn-id: svn://svn.chromium.org/blink/trunk@181454 bbb929c8-8fbe-4397-9dbb-9b2b20218538