config: can't keep existing `SecretAttribute` value with Return key

[dgw]

In the config wizard invoked by e.g. sopel-plugins configure pluginname, many options that have a default value or have already been configured can be "skipped" (keeping the current value) by pressing Return without typing a new value. But for SecretAttribute, trying to keep the existing value will display an error. Here's a real example I ran into earlier this week:

$ sopel-plugins configure bsky
Configure sopel-bsky
Bluesky account handle: [bots-current-handle.bsky.social]
Bluesky account password: (hidden input)
You must provide a value for this option.
Bluesky account password: (hidden input)

This stymied my testing a bit. (Ultimately I just commented out the preexisting settings so I could test the new stuff.)