VBucketBackupWriter should correctly handle an empty stats file

In the event of an os crash (while testing an unrelated bug I inserted an os.Exit() to simulate a crash) it's possible for a stats file to have been created, but the contents were not marshalled and written in time. This will trigger a branch on the resume which will try to read the old stats file before adding the additional stats information.

Whats' the fix?
When we do the 'Stat' call to check if the file exists, we should check if the size is zero, if it is, we should just write the in-memory stats and not bother "appending" our stats information.