Bugfix: Make snapshots --json output [] instead of null when no snapshots

Restic previously output `null` instead of `[]` for the `--json snapshots`
command, when there were no snapshots in the repository. This caused some
minor problems when parsing the output, but is now fixed such that `[]` is
output when the list of snapshots is empty.

https://github.com/restic/restic/issues/2979
https://github.com/restic/restic/pull/2984
