Enhancement: Support repositories with empty password

Restic refused to create or operate on repositories with an empty password.
Using the new option `--insecure-no-password` it is now possible to disable
this check. Restic will not prompt for a password when using this option.
For security reasons, the option must always be specified when operating on
repositories with an empty password.

Specifying `--insecure-no-password` while also passing a password to restic
via a CLI option or via environment variable results in an error.

The `init` and `copy` command also support the option `--from-insecure-no-password`
which applies to the source repository. The `key add` and `key passwd` commands
include the `--new-insecure-no-password` option to add or set an empty password.

https://github.com/restic/restic/issues/1786
https://github.com/restic/restic/issues/4326
https://github.com/restic/restic/pull/4698
https://github.com/restic/restic/pull/4808