Enhancement: Ignore empty lock files

With restic 0.15.0 the checks for stale locks became much stricter than before.
In particular, empty or unreadable locks were no longer ignored. This caused
restic to complain about `Load(<lock/1234567812>, 0, 0) returned error,
retrying after 552.330144ms: load(<lock/1234567812>): invalid data returned`
and fail in the end.

We have clarified the error message and changed the implementation to ignore
empty lock files which are sometimes created as the result of a failed upload
on some backends. Unreadable lock files still have to cleaned up manually. To
do so, you can run `restic unlock --remove-all` which removes all existing lock
files. But first make sure that no other restic process is currently running.

https://github.com/restic/restic/issues/4143
https://github.com/restic/restic/pull/4152