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 silently ignored. This
made restic to complain with `Load(<lock/1234567812>, 0, 0) returned error,
retrying after 552.330144ms: load(<lock/1234567812>): invalid data returned`
and fail in the end.

The error message is now clarified and the implementation changed to ignore
empty lock files which are sometimes created as the result of a failed uploads
on some backends.

Please note that 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 using the
repository.

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