Enhancement: Support filtering and specifying untagged snapshots

It was previously not possible to specify an empty tag with the `--tag` and
`--keep-tag` options. This has now been fixed, such that `--tag ''` and
`--keep-tag ''` now matches snapshots without tags. This allows e.g. the
`snapshots` and `forget` commands to only operate on untagged snapshots.

https://github.com/restic/restic/issues/3456
https://github.com/restic/restic/pull/3457
